Declare constants at the native precision of MPAS

[kuanchihwang]

When building MPAS as a dynamical core, certain constants in the mpas_constants module are imported from the physconst module, which is a part of CAM/CAM-SIMA. However, multiple issues arise if the precision of those constants differs from MPAS.

For example, building MPAS in single precision mode with CAM-SIMA fails due to multiple occurrences of type mismatch between actual and dummy arguments.

mpas_geometry_utils.F:885:157:

  885 |          call mpas_log_write('$r', MPAS_LOG_ERR, realArgs=(/mpas_triangle_signed_area_sphere(a,b,c,sphereRadius) - pii/2.0_RKIND*sphereRadius*sphereRadius/))
      |                                                                                                                                                             1
Error: Type mismatch in argument 'realargs' at (1); passed REAL(8) to REAL(4)

Here, pii is declared by CAM-SIMA to be double precision, and it causes unintended floating-point promotion in the expression.

The solution is to ensure that constants in the mpas_constants module are declared at the native precision of MPAS. Note that CAM does not support running MPAS in single precision mode at all, so the above build error cannot be reproduced.

In addition, some best practices of modern Fortran have been implemented.

For stand-alone MPAS, the compiled executables stay bitwise identical, with or without this PR. No further tests are needed. For CAM/CAM-SIMA, it generates bitwise identical model results, with or without this PR. Additionally for CAM-SIMA, its regression tests all pass, indicating identical model results to the previous baseline.

[abishekg7]

Overall looks good. I did have a few comments and question though.

[abishekg7]

Looks good!

[mgduda]

@kuanchihwang The code changes themselves look good. However, I'd like to request a few minor changes to the commit messages so that they more specifically describe what is being modified. For example, the commit messages

Enforce implicit none at the module level

Enforce `implicit none` at the module level so it applies everywhere
within. It is also a best practice in Fortran.

and

Trim trailing whitespaces

give no indication of what code is affected.

Could you revisit the commit messages and ensure that they make it clear what module, file, or part of the model is being affected? I'm not suggesting we add a list of modified files to each commit message, but rather, reference module or file names in the text. For example, the last commit message I mentioned above could become:

Clean up whitespace in the mpas_constants module

Several instances of trailing spaces have been removed, and a blank line has
been added above the 'contains' statement in the mpas_constants module for
consistency with other MPAS modules.

[kuanchihwang]

I have amended the commit messages to include more specific information about what they modify. The commit contents remain the same as can be seen by comparison.