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.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

:exclamation: 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

[](https://app.codecov.io/gh/cmelab/grits/pull/74?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab) ```diff @@ Coverage Diff @@ ## main #74 +/- ## ======================================= Coverage 95.14% 95.14% ======================================= Files 4 4 Lines 474 474 ======================================= Hits 451 451 Misses 23 23 ``` | [Files](https://app.codecov.io/gh/cmelab/grits/pull/74?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab) | Coverage Δ | | |---|---|---| | [grits/coarsegrain.py](https://app.codecov.io/gh/cmelab/grits/pull/74?src=pr&el=tree&filepath=grits%2Fcoarsegrain.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cmelab#diff-Z3JpdHMvY29hcnNlZ3JhaW4ucHk=) | `95.94% <100.00%> (ø)` | |