Move hoomd import inside rigid body function

[chrisjonesBSU]

This is a small change that moves the hoomd import from the top of gsd_utils.py into the create_rigid_snapshot method, which is the only place it is used.

Importing and using other cmeutils methods was causing issues on clusters when my environment had the gpu-version of hoomd, but the job wasn't on a gpu node.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dca592a) 96.28% compared to head (42d639f) 96.28%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #80   +/-   ##
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  Coverage   96.28%   96.28%           
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  Files           8        8           
  Lines         781      781           
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  Hits          752      752           
  Misses         29       29           

Files	Coverage Δ
cmeutils/gsd_utils.py	91.66% <100.00%> (ø)