Add tests for dynamic_mask and remove usage of numpy.where

[braingram]

This PR adds some basic unit tests for dynamic_mask and improves performance by removing use of np.where calls on binary masks and instead uses the masks directly (local testing shows the new code takes ~30% the time).

Regtest run with these changes show no errors:
https://plwishmaster.stsci.edu:8081/blue/organizations/jenkins/RT%2FJWST-Developers-Pull-Requests/detail/JWST-Developers-Pull-Requests/812/pipeline/123

Checklist

• added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03% ⚠️

Comparison is base (308f828) 64.04% compared to head (746546c) 64.01%.
Report is 1 commits behind head on master.

❗ Current head 746546c differs from pull request most recent head 1abd64e. Consider uploading reports for the commit 1abd64e to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #185      +/-   ##
==========================================
- Coverage   64.04%   64.01%   -0.03%     
==========================================
  Files         101      101              
  Lines        5562     5558       -4     
==========================================
- Hits         3562     3558       -4     
  Misses       2000     2000              

Files Changed	Coverage Δ
src/stdatamodels/dynamicdq.py	100.00% <100.00%> (ø)

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