Adding unit tests for functions that use MPI

[wehs7661]

In this PR, we complete the work in test_mpi_func.py, which tests functions that use MPI, including _run_grompp, _run_mdrun, and run_REXEE. Except for the tests itself, some key things were done, including

• Adding a synchronization point done by comm.barrier, which presumably prevented some (uninformative) errors from MPI_Abort().
• Editing the configuration file setup.cfg by adding parallel=True, which prevented the "internal errors" as a result of race conditions with file access between MPI processes.
• Using coverage instead of pytest, as coverage seems to work better with MPI, especially with the configuration file explicitly passed.

[codecov[bot]]

Codecov Report

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

Project coverage is 95.56%. Comparing base (a0f93b7) to head (0c8a668).

:exclamation: Current head 0c8a668 differs from pull request most recent head 3aa87c2. Consider uploading reports for the commit 3aa87c2 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #40 +/- ## ========================================== + Coverage 92.34% 95.56% +3.22% ========================================== Files 8 8 Lines 1645 1646 +1 ========================================== + Hits 1519 1573 +54 + Misses 126 73 -53 ```

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