fixes nan error in fast powder calibration

[saransh13]

ValueError: cannot convert float NaN to integer error was generated when running fast powder calibration on some configurations. Culprit was found to be some invalid pixels when searching for peaks. Masking out those peaks before searching.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 39.61%. Comparing base (dfbeb97) to head (ffa8ace).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
hexrd/instrument/hedm_instrument.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #754      +/-   ##
==========================================
- Coverage   39.62%   39.61%   -0.01%     
==========================================
  Files         140      140              
  Lines       22696    22701       +5     
==========================================
  Hits         8994     8994              
- Misses      13702    13707       +5     

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