REFACTOR: make multiband likelihood call Interferometer.get_detector_response

Based on ongoing discussion about implementing different detector responses, it would be nice for the likelihoods to all access the response functions uniformly. The MB likelihood currently has an almost verbatim copy of the Inteferometer.get_detector_response method. This MR provides a minimal change to be able to directly call that method.

@pratyusava.baral would this simplify your modification of the MB likelihood?