Restructure the detector module

This is the first step at restructuring this module. I moved everything into a detector/ subdirectory. Everything is structured so that the API remains backwards compatible.

bilby/gw/detector/strain_data.py

@@ -15,7 +15,6 @@ except ImportError:
 
 try:
     import lal
-    import lalsimulation as lalsim
 except ImportError:
     logger.warning("You do not have lalsuite installed currently. You will"
                    " not be able to use some of the prebuilt functions.")