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/interferometer.py

@@ -930,4 +930,4 @@ class Interferometer(object):
         res = deepdish.io.load(filename)
         if res.__class__ != cls:
             raise TypeError('The loaded object is not an Interferometer')
-        return res
\ No newline at end of file
+        return res