numba accelerated special functions

[saransh13]

a few of the functions such as exp1 implemented previously were not correct for complex arguments. The latest implementation fixes that.

Also included is a new special function wofzwhich implements the Faddeeva function for complex arguments.

Both these are needed for the new peak shapes to deal with x-ray induced heating experiments.

The PR removes some unused constants in the herd.core.constants and moves all the special function implementation to hexrd.core.fitting.special. The paths in hexrd.powder.wppf.peakfunctions and hexed.core.fitting.peakfunctions have been updated to reflect this move.

Also added tests for these new functions by comparing them to spicy.special.

[codecov]

Codecov Report

❌ Patch coverage is 97.79736% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.65%. Comparing base (4e73408) to head (7a42bfd).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
hexrd/powder/wppf/peakfunctions.py	33.33%	4 Missing ⚠️
hexrd/core/fitting/special.py	99.53%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #942      +/-   ##
==========================================
+ Coverage   72.62%   72.65%   +0.03%     
==========================================
  Files         142      143       +1     
  Lines       21940    22085     +145     
==========================================
+ Hits        15933    16045     +112     
- Misses       6007     6040      +33     

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