Adds back in injection_polarization argument

- Provides backward compatibility for people with scripts using the old
  style
- Also because, as Colm pointed out, there may be bugger behaviour with
  multiple detectors and it may take a long time to calculate the waveform
- Change the name in `inject_signal` from
  waveform_polarizations->injection_polarizations.

This last step could break peoples code who are using inject_signal
directly, but its probably best to unify notation now rather than have
two names for one thing.