Modify inject signal

This restructures some functionality surrounding the inject_signal method.

• Added inject_signal_from_waveform_generator and inject_signal_from_waveform_polarizations that are slightly less confusing to use than inject_signal.
• Removed if/else statement that checks size of the injected signal. The injection will now just fail if that condition is now not fulfilled. It was already breaking before, but at a different place.
• Added explicit tests for these methods.

bilby/gw/detector/interferometer.py

@@ -298,12 +298,12 @@ class Interferometer(object):
         ----------
         parameters: dict
             Parameters of the injection.
-        injection_polarizations: dict
+        injection_polarizations: dict, optional
            Polarizations of waveform to inject, output of
            `waveform_generator.frequency_domain_strain()`. If
            `waveform_generator` is also given, the injection_polarizations will
            be calculated directly and this argument can be ignored.
-        waveform_generator: bilby.gw.waveform_generator.WaveformGenerator
+        waveform_generator: bilby.gw.waveform_generator.WaveformGenerator, optional
             A WaveformGenerator instance using the source model to inject. If
             `injection_polarizations` is given, this will be ignored.
 
@@ -316,6 +316,8 @@ class Interferometer(object):
         Returns
         -------
         injection_polarizations: dict
+            The injected polarizations. This is the same as the injection_polarizations parameters
+            if it was passed in. Otherwise it is the return value of waveform_generator.frequency_domain_strain().
 
         """
         if injection_polarizations is None and waveform_generator is None: