Add method to get data by channel name

This addresses the issues encountered when trying to load data spanning multiple frame files, and adopts the bilby_pipe method of loading data using gwpy.TimeSeries.get. I have added several functions along the lines of set_from_channel_name (I'm open to other suggestions for what to call this method) for creating an InterferometerStrainData object, a PowerSpectralDensity object, setting the interferometer.strain_data, and for creating an Interferometer object from the channel name directly. I basically followed the template of the from_frame_file functions for the structure. I also edited the load_from_cache_file function to address the logic issues. A couple questions:

1. Should I make a separate gw.utils function for the actual loading of the data using gwpy.TimeSeries.get()? It's two lines so it seemed like overkill to add another function just for this instead of doing it directly in the set_from_channel_name method of the strain_data object.
2. Should I add a buffer time? This is an argument of the from_frame_file function but it doesn't seem to be used... (https://git.ligo.org/lscsoft/bilby/blob/master/bilby/gw/utils.py#L387)

If you want to check out some plots to make sure this is doing what it's supposed to, I've plotted the PSDs estimated using this new method, the time domain strain, and the frequency domain strain (along with the Bayeswave ASD) for one trigger here: https://ldas-jobs.ligo.caltech.edu/~sylvia.biscoveanu/O3/S190521g/

bilby/gw/detector/__init__.py

@@ -233,7 +233,13 @@ def load_data_from_cache_file(
     psd_set = False
 
     with open(cache_file, 'r') as ff:
-        for line in ff:
+        lines = ff.readlines()
+        if len(lines)>1:
+            raise ValueError('This method cannot handle cache files with'
+                             ' multiple frames. Use `load_data_by_channel_name'
+                             ' instead.')
+        else:
+            line = lines[0]
             cache = lal.utils.cache.CacheEntry(line)
             data_in_cache = (
                 (cache.segment[0].gpsSeconds < start_time) &
@@ -243,7 +249,13 @@ def load_data_from_cache_file(
                 (cache.segment[1].gpsSeconds > psd_start_time + psd_duration))
             ifo = get_empty_interferometer(
                 "{}1".format(cache.observatory))
-            if not data_set & data_in_cache:
+            if not data_in_cache:
+                raise ValueError('The specified data segment does not exist in' 
+                                 ' this frame.')
+            if not psd_in_cache:
+                raise ValueError('The specified PSD data segment does not exist' 
+                                 ' in this frame.')
+            if (not data_set) & data_in_cache:
                 ifo.set_strain_data_from_frame_file(
                     frame_file=cache.path,
                     sampling_frequency=sampling_frequency,
@@ -251,7 +263,7 @@ def load_data_from_cache_file(
                     start_time=start_time,
                     channel=channel_name, buffer_time=0)
                 data_set = True
-            if not psd_set & psd_in_cache:
+            if (not psd_set) & psd_in_cache:
                 ifo.power_spectral_density = \
                     PowerSpectralDensity.from_frame_file(
                         cache.path,
@@ -272,3 +284,64 @@ def load_data_from_cache_file(
         raise ValueError('Data not loaded for {}'.format(ifo.name))
     elif not psd_set:
         raise ValueError('PSD not created for {}'.format(ifo.name))
+
+
+def load_data_by_channel_name(
+        channel_name, start_time, segment_duration, psd_duration, psd_start_time,
+        sampling_frequency=4096, roll_off=0.2,
+        overlap=0, outdir=None):
+    """ Helper routine to generate an interferometer from a channel name
+    This function creates an empty interferometer specified in the name 
+    of the channel. It calls `ifo.set_strain_data_from_channel_name` to 
+    set the data and PSD in the interferometer using data retrieved from 
+    the specified channel using gwpy.TimeSeries.get()
+
+    Parameters
+    ----------
+    channel_name: str
+        Channel name with the format `IFO:Channel`
+    start_time, psd_start_time: float
+        GPS start time of the segment and data stretch used for the PSD
+    segment_duration, psd_duration: float
+        Segment duration and duration of data to use to generate the PSD (in
+        seconds).
+    roll_off: float, optional
+        Rise time in seconds of tukey window.
+    overlap: float,
+        Number of seconds of overlap between FFTs.
+    sampling_frequency: int
+        Sampling frequency
+    outdir: str, optional
+        The output directory in which the data is saved
+
+    Returns
+    -------
+    ifo: bilby.gw.detector.Interferometer
+        An initialised interferometer object with strain data set to the
+        appropriate data fetched from the specified channel and a PSD.
+    """
+    try:
+        det = channel_name.split(':')[-2]
+    except IndexError:
+        raise IndexError("Channel name must be of the format `IFO:Channel`")
+    ifo = get_empty_interferometer(det)
+        
+    ifo.set_strain_data_from_channel_name(
+        channel = channel_name,
+        sampling_frequency=sampling_frequency,
+        duration=segment_duration,
+        start_time=start_time)
+    
+    ifo.power_spectral_density = \
+        PowerSpectralDensity.from_channel_name(
+            channel=channel_name,
+            psd_start_time=psd_start_time,
+            psd_duration=psd_duration,
+            fft_length=segment_duration,
+            sampling_frequency=sampling_frequency,
+            roll_off=roll_off,
+            overlap=overlap,
+            name=det,
+            outdir=outdir,
+            analysis_segment_start_time=start_time)
+    return ifo