Rename the EEXE method as REXEE method

[wehs7661]

We decided to rename the EEXE method as REXEE, which stands for replica exchange (REX) of expanded ensembles (EE). This requires updating the names of some modules and classes, as well as the docstrings and the documentation.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (9cb910f) 55.09% compared to head (a95ea2d) 55.09%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #27   +/-   ##
=======================================
  Coverage   55.09%   55.09%           
=======================================
  Files           7        7           
  Lines        1552     1552           
=======================================
  Hits          855      855           
  Misses        697      697           

Files	Coverage Δ
ensemble_md/analysis/analyze_matrix.py	99.08% <ø> (ø)
ensemble_md/replica_exchange_EE.py	74.13% <100.00%> (ø)
ensemble_md/utils/gmx_parser.py	85.86% <ø> (ø)
ensemble_md/utils/utils.py	63.44% <100.00%> (ø)
ensemble_md/analysis/analyze_traj.py	5.49% <50.00%> (ø)

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