Refactor strain data

Part 3 in the detector overhaul. Builds on !473 (merged)

This generalizes the use of the PropertyAccessor to InterferometerStrainData and WaveformGenerator

Keeps everything nice and backwards compatible

bilby/gw/detector/__init__.py

@@ -3,6 +3,7 @@ from .interferometer import *
 from .networks import *
 from .psd import *
 from .strain_data import *
+from .calibration import *
 
 try:
     import lal