Fixed GPU with NVHPC 25.3 - f_tri_area

[okBrian]

Description

When building with NVHPC 25.3, I was getting a FORT 2 TERMINATED by SIGNAL 11. The cause of the error was the function f_tri_area, It seemed to be fixed by changing it into a subroutine.

Fixes #(issue) [optional]

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Something else

Scope

• This PR comprises a set of related changes with a common goal

If you cannot check the above box, please split your PR into multiple PRs that each have a common goal.

How Has This Been Tested?

• Built Locally with NVHPC 25.3
• Built on Phoenix NVHPC 24.7

Test Configuration:

• What computers and compilers did you use to test this:
  Locally w/ Linux - NVHPC 25.3
  Phoenix NVHPC 24.7

Checklist

• I ran ./mfc.sh format before committing my code
• New and existing tests pass locally with my changes, including with GPU capability enabled (both NVIDIA hardware with NVHPC compilers and AMD hardware with CRAY compilers) and disabled
• This PR does not introduce any repeated code (it follows the DRY principle)
• I cannot think of a way to condense this code and reduce any introduced additional line count

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.

Project coverage is 42.95%. Comparing base (db44da1) to head (9397372).

Files with missing lines	Patch %	Lines
src/pre_process/m_model.fpp	0.00%	11 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #873   +/-   ##
=======================================
  Coverage   42.95%   42.95%           
=======================================
  Files          69       69           
  Lines       19504    19504           
  Branches     2366     2366           
=======================================
  Hits         8377     8377           
  Misses       9704     9704           
  Partials     1423     1423           

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