Add sflux forcing from existing file functionality for SCHISM setup

[SorooshMani-NOAA]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 35.82090% with 43 lines in your changes missing coverage. Please review.

Project coverage is 64.53%. Comparing base (6a97a9a) to head (f97983c).
Report is 20 commits behind head on main.

Files	Patch %	Lines
coupledmodeldriver/configure/forcings/base.py	32.81%	43 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
- Coverage   64.84%   64.53%   -0.31%     
==========================================
  Files          35       35              
  Lines        3869     3934      +65     
==========================================
+ Hits         2509     2539      +30     
- Misses       1360     1395      +35     

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