Add Interferometer class

I added Interferometer class in detector.py. That is inherited from bilby.gw.detector.Interferometer and have an additional classmethod to initialize its class for the given strain data info. The initialization can be done without setting gwpy.Timeseries object for the testing purpose. Parsing the ifo geometry information is currently separated as a staticmethod get_geometry_kwargs() so that that function can be called somewhere else in case one wants to create another classmethod for different way of the class initialization in the future. That said, if that is not necessary, the function can be incorporated inside get_interferometer().

@alba.romero Please take a look at the way that this class interfaces with pre_processing.py and let me know if that makes sense. Also, once this is merged, could you try to add a functionality that stores fftgram object for the given gwpy.timeseries object and that deletes the raw timeseries data once the fftgram is stored.