Adding more unit tests for the package

[wehs7661]

This PR aims to address parts of the todos listed in #41, including the following action items in the section of "Improving the code coverage":

• [x] Add unit tests for analysis/analyze_free_energy.py
• [x] Add unit tests for analysis/clustering.py
• [x] Add unit tests for analysis/synthesize_data.py
• [x] Complete test_mpi_func.py, which tests functions that use MPI

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.34146% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 96.10%. Comparing base (f5daa68) to head (27320da).

Files	Patch %	Lines
ensemble_md/analysis/clustering.py	95.16%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #42 +/- ## ========================================== + Coverage 95.56% 96.10% +0.53% ========================================== Files 8 10 +2 Lines 1646 1950 +304 ========================================== + Hits 1573 1874 +301 - Misses 73 76 +3 ```

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