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

@@ -346,8 +346,8 @@ class Interferometer(object):
         Note
         -------
         if your signal takes a substantial amount of time to generate, or
-        you experience buggy behaviour. It is preferable to provide the
-        injection_polarizations directly.
+        you experience buggy behaviour. It is preferable to use the
+        inject_signal_from_waveform_polarizations() method.
 
         Returns
         -------