Fix issues for using NWM

[SorooshMani-NOAA]

• After adding cache files for NWM, the default object type conversion runs into an issue for PathLike type.
• The docstring was missing from the NWM configuration class definition

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.84%. Comparing base (25a599e) to head (da42960).
Report is 61 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #152   +/-   ##
=======================================
  Coverage   64.84%   64.84%           
=======================================
  Files          35       35           
  Lines        3871     3871           
=======================================
  Hits         2510     2510           
  Misses       1361     1361           

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