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

 from bilby.gw import conversion
+from .calibration import *
 from .interferometer import *
 from .networks import *
 from .psd import *