Check and enforce FLOAT64 snapshots for null dycore

[jimmielin]

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Originator(s): @jimmielin co-authored with claude-opus-4.6

Description (include the issue title, and the keyword ['closes', 'fixes', 'resolves'] followed by the issue number):

• Enforce 64-bit snapshots being read into the null dycore
  If the snapshot is not written out with ndens=1 on the CAM end it will cause incomprehensible answer differences comparing CAM and CAM-SIMA. So if a FLOAT32 snapshot is detected now the model will crash:

dec2442.hsn.de.hpc.ucar.edu 0:  model_grid_init: Snapshot file has non-FLOAT64 data (variable state_u). This will cause answer differences!Please rerun CAM with ndens = 1 to write 64-bit float snapshots for use with CAM-SIMA.

This only applies when ncdata_check is set, in case the same null dycore functionality is used for non-CAM snapshot runs (thanks to @nusbaume for the suggestion)

Describe any changes made to build system: N/A

Describe any changes made to the namelist: N/A

List any changes to the defaults for the input datasets (e.g. boundary datasets): N/A

List all files eliminated and why: N/A

List all files added and what they do: N/A

List all existing files that have been modified, and describe the changes:
(Helpful git command: git diff --name-status development...<your_branch_name>)

M       src/dynamics/none/dyn_grid.F90
  - add a PIO_DOUBLE check on the existing check for U/state_u/eastward_wind

M.      src/physics/phys_comp.F90
  - make ncdata_check public for access by dyn_grid.F90.

If there are new failures (compared to the test/existing-test-failures.txt file),
have them OK'd by the gatekeeper, note them here, and add them to the file.
If there are baseline differences, include the test and the reason for the
diff. What is the nature of the change? Roundoff?

derecho/intel/aux_sima:

derecho/gnu/aux_sima:

derecho/nvhpc/aux_sima:

If this changes climate describe any run(s) done to evaluate the new
climate in enough detail that it(they) could be reproduced:

CAM-SIMA date used for the baseline comparison tests if different than latest:

[nusbaume]

Thanks for the additional check @jimmielin! I just had one change request, which is basically that this check should only be done if we know we are running a snapshot test by examining the ncdata_check namelist variable as well.

[nusbaume]

Thanks @jimmielin! Everything looks good to me now, although I did have one (optional) variable declaration order request.