some fixes for simulating powder diffraction pattern on an instrument.

[saransh13]

Spotted two errors:

1. the size of stacking fault shifts and stacking fault broadening not the same
2. the angularpixelsize has a zeros which was taken as the angular resolution. masking out zeros in those values.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.13%. Comparing base (e8257a7) to head (cbe0861).
Report is 73 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #773      +/-   ##
==========================================
+ Coverage   46.12%   46.13%   +0.01%     
==========================================
  Files         142      142              
  Lines       22822    22828       +6     
==========================================
+ Hits        10526    10532       +6     
  Misses      12296    12296              

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