Fix distortion check

[psavery]

It needed to check an object one level further down...

[codecov]

Codecov Report

Attention: Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.

Project coverage is 48.35%. Comparing base (0cc4340) to head (9df499c).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
hexrd/instrument/detector.py	0.00%	3 Missing ⚠️
hexrd/fitting/calibration/abstract_grain.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #784      +/-   ##
==========================================
+ Coverage   48.20%   48.35%   +0.14%     
==========================================
  Files         143      143              
  Lines       22935    22860      -75     
==========================================
- Hits        11056    11054       -2     
+ Misses      11879    11806      -73     

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