Phase transformation

[saransh13]

Overview

This PR is in preparation for diffraction simulations during phase transformation. The implemented function computes the phase transformation variants given an orientation relationship between the parent and daughter phases. The function has a handy keyword for plotting the pole figures for the primary, secondary and tertiary symmetry directions of the two phases. The orientation for the parent phase for the plotting is always the identity matrix.

This PR has introduced a new dependency mplstereonet used in the plotting routine.

The next set of PRs will use this information, along with MTEX texture functions to do a forward simulation of diffraction signal across a phase transformation boundary.

[codecov]

Codecov Report

❌ Patch coverage is 50.92025% with 80 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.09%. Comparing base (f18192f) to head (c7799c4).

Files with missing lines	Patch %	Lines
hexrd/singlextal/phasetransformation/variants.py	61.11%	49 Missing ⚠️
hexrd/core/material/spacegroup.py	3.22%	30 Missing ⚠️
hexrd/core/valunits.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #854      +/-   ##
==========================================
+ Coverage   56.01%   56.09%   +0.07%     
==========================================
  Files         143      144       +1     
  Lines       22023    22186     +163     
==========================================
+ Hits        12337    12445     +108     
- Misses       9686     9741      +55     

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