Only pass one rMat_s to angularPixelSize

[psavery]

We were previously passing an array of matrices, and only the first matrix was being used in the old transforms code. In the new transforms code, an error is being raised because it expects only a single matrix (and not an array of matrices). We should explicitly only pass this first matrix to angularPixelSize().

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 39.69%. Comparing base (d64428f) to head (fa63866).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
hexrd/xrdutil/utils.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #738      +/-   ##
==========================================
- Coverage   39.69%   39.69%   -0.01%     
==========================================
  Files         135      135              
  Lines       22380    22383       +3     
==========================================
+ Hits         8884     8885       +1     
- Misses      13496    13498       +2     

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[psavery]

This PR maintains the previous behavior when we were using the old transforms.

However, I think it's possible that the rMat_s for the angular pixel size doesn't actually matter, and we can just set it to identity.