Updating docstrings to latest flake8 version

[marcomangano]

Purpose

Minor changes to the docstrings of a few files. I also added a brief description of the FluidProperties class.

Expected time until merged

ASAP, it will fix the failed style checks for #89

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

Checklist

• I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation

[codecov]

Codecov Report

Merging #90 (e198ab9) into main (1281ec4) will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   35.39%   35.44%   +0.05%     
==========================================
  Files          26       26              
  Lines        2582     2584       +2     
==========================================
+ Hits          914      916       +2     
  Misses       1668     1668              

Impacted Files	Coverage Δ
baseclasses/problems/FluidProperties.py	10.00% <ø> (ø)
baseclasses/problems/pyFieldPerformance_problem.py	16.25% <ø> (ø)
baseclasses/testing/pyRegTest.py	87.95% <ø> (ø)

... and 2 files with indirect coverage changes

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