Made calibration shift backwards compatible

bilby/gw/__init__.py

@@ -3,5 +3,5 @@ from . import (conversion, cosmology, detector, likelihood, prior,
 
 from .waveform_generator import WaveformGenerator
 from .likelihood import GravitationalWaveTransient
-
+from .detector import calibration
 

bilby/gw/detector/__init__.py

 from bilby.gw import conversion
+from . import calibration
 from .interferometer import *
 from .networks import *
 from .psd import *
 from .strain_data import *
-from .calibration import *
 
 try:
     import lal

test/calibration_test.py

-from bilby.gw.detector import calibration
+from bilby.gw import calibration
 import unittest
 import numpy as np