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/__init__.py

 from bilby.core.utils import logger
-from ...gw import conversion
-from .detector import *
+from .. import conversion
 from .interferometer import *
 from .networks import *
 from .psd import *
@@ -479,4 +478,4 @@ def inject_signal_into_gwpy_timeseries(
     for key in parameters:
         logger.info('  {} = {}'.format(key, parameters[key]))
 
-    return signal_and_data, meta_data
\ No newline at end of file
+    return signal_and_data, meta_data