Compton scattering

[saransh13]

Add functionality to compute Compton scattering intensity on an instrument panel. This is useful for intensity corrections during liquid total scattering analysis. There were two main calculation needed for this:

1. absorption of the inelastically scattered photons by the sample and window. since the inelastically scattered photons lose energy, the attenuation length is angle dependent.
2. computation of incoherent scattering intensity for each detector pixel. this is done by interpolating the table in resources/Anomalous.h5.
3. helper function in `material.utils

There is some special handling needed when the window is None or the thickness is 0, both of which are valid ways to omit the window from the calculation.

[codecov]

Codecov Report

Attention: Patch coverage is 19.20530% with 122 lines in your changes missing coverage. Please review.

Project coverage is 46.48%. Comparing base (dec6393) to head (e90fd6f).
Report is 24 commits behind head on master.

Files with missing lines	Patch %	Lines
hexrd/instrument/detector.py	16.66%	75 Missing ⚠️
hexrd/material/utils.py	21.66%	47 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #775      +/-   ##
==========================================
+ Coverage   45.12%   46.48%   +1.35%     
==========================================
  Files         142      142              
  Lines       22822    22970     +148     
==========================================
+ Hits        10298    10677     +379     
+ Misses      12524    12293     -231     

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