Load data by channel name example

This MR adds an example to demonstrate the new method of loading data that was merged in !507 (merged). It copies heavily from the read_gracedb_data.py example, so I'm not sure how people feel about deleting that one at some point.

examples/open_data_examples/read_data_from_channel_name.py

+#!/usr/bin/env python
+"""
+Tutorial to demonstrates reading and plotting the GraceDB data for GW170608,
+the lightest GWTC-1 BBH event.
+
+This tutorial must be run on a LIGO cluster.
+"""
+
+import bilby
+
+label = 'GW170608'
+outdir = label + '_out'
+# List of detectors involved in the event. GW170608 is pre-August 2017 so only
+# H1 and L1 were involved
+detectors = ['H1', 'L1']
+# Names of the channels and query types to use, and the event GraceDB ID - ref.
+# https://ldas-jobs.ligo.caltech.edu/~eve.chase/monitor/online_pe/C02_clean/
+# C02_HL_Pv2/lalinferencenest/IMRPhenomPv2pseudoFourPN/config.ini
+channel_names = ['H1:DCH-CLEAN_STRAIN_C02', 'L1:DCH-CLEAN_STRAIN_C02']
+gracedb = 'G288686'
+# Duration of data around the event to use
+duration = 16
+# Duration of PSD data
+psd_duration = 1024
+# Minimum frequency and reference frequency
+minimum_frequency = 10  # Hz
+sampling_frequency = 2048.
+
+# Get trigger time
+candidate = bilby.gw.utils.gracedb_to_json(gracedb, outdir=outdir)
+trigger_time = candidate['gpstime']
+gps_start_time = trigger_time + 2. - duration
+
+# Load the PSD data starting after the segment you want to analyze
+psd_start_time = gps_start_time + duration
+
+# Set up interferometer objects from the cache files
+interferometers = bilby.gw.detector.InterferometerList([])
+
+for channel_name in channel_names:
+    interferometer = bilby.gw.detector.load_data_by_channel_name(
+        start_time=gps_start_time, segment_duration=duration,
+        psd_duration=psd_duration, psd_start_time=psd_start_time,
+        channel_name=channel_name, sampling_frequency=sampling_frequency, outdir=outdir)
+    interferometer.minimum_frequency = minimum_frequency
+    interferometers.append(interferometer)
+
+# Plot the strain data
+interferometers.plot_data(outdir=outdir, label=label)