move around the detector response to the signal into a new function
This merge request removes the duplicate code from likelihood/py.
It introduces a new function Interferometer.get_detector_response which takes the polarizations of a waveform
and projects them onto the interferometer.
This requires passing a subset of the parameters (the ones defining the detector response) to the new function.
This changes the signal injection syntax.
I think this fits into a wider question of how we want to pass around the ra, dec, time and phase of the waveform which don't change the polarizations.