MT-REXEE development

[wehs7661]

Merge PR-57 into the master branch to incorporate final tweaks and updates based on the original Anika's pull request (#57). Here is a checklist for the PR to be merged:

• [x] Enough unit tests to achieve a 95% code coverage (by PR #59).
• [x] No lining errors.
• [x] No errors from CircleCI.
• [x] Address issue #60.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.45127% with 17 lines in your changes missing coverage. Please review.

Project coverage is 96.00%. Comparing base (9958558) to head (98af948). Report is 77 commits behind head on master.

Files with missing lines	Patch %	Lines
ensemble_md/utils/coordinate_swap.py	97.87%	11 Missing :warning:
ensemble_md/replica_exchange_EE.py	96.90%	3 Missing :warning:
ensemble_md/utils/gmx_parser.py	94.33%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #58 +/- ## ========================================== + Coverage 95.51% 96.00% +0.48% ========================================== Files 10 11 +1 Lines 1919 2576 +657 ========================================== + Hits 1833 2473 +640 - Misses 86 103 +17 ```

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[wehs7661]

Hey @ajfriedman22 I have just closed #57 and #58, so we can just keep throwing everything about MT-REXEE into this PR (e.g., unit tests, or any necessary tweaks for the core functionalities). For documentation for MT-REXEE, it might be better to have a separate branch for it, so we can prioritize merging this into the master branch. Also, it would be great if you could help resolve the linting and CI errors. You can find more information in the contributing guidelines: https://github.com/wehs7661/ensemble_md/blob/master/CONTRIBUTING.md