Add stride parameter when saving coarse-grained trajectories

[chrisjonesBSU]

This PR adds a stride parameter that can be used when saving coarse-grain trajectories. This should we helpful when we have large, long atomistic trajectories and don't need every frame in the coarse-grained trajectory.

[marjanalbooyeh]

LGTM. Thank you Chris!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.14%. Comparing base (ff9f81d) to head (c3ba3cc).
Report is 10 commits behind head on main.

❗ Current head c3ba3cc differs from pull request most recent head 29e570d. Consider uploading reports for the commit 29e570d to get more accurate results

Additional details and impacted files

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  Coverage   95.14%   95.14%           
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  Files           4        4           
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  Hits          451      451           
  Misses         23       23           

Files	Coverage Δ
grits/coarsegrain.py	95.94% <100.00%> (ø)