Allow diag_table changes via user_nl_mom, and fix user_nl_mom/MOM_override bugs

[alperaltuntas]

Allow changing the diag_table via user_nl_mom, and fix several bugs in how user_nl_mom is read and turned into MOM_override.

diag_table changes are given as diag_table%<stream>%<key> <operator> <value>, e.g.:

    diag_table%h.native%fields += MLD_0125, Kd_heat:Kd_heat_max:max
    diag_table%h.native%fields -= KE
    diag_table%h.sfc%output_freq = 5
    diag_table%h.z%fields = none
    diag_table%h.mine%module = ocean_model_z
    diag_table%h.mine%fields = thetao, so

• Changes are applied when diag_table.unresolved is generated, so both the unresolved and resolved diag_tables include them.
• Settings act as if given in the template entry, so derived settings follow (e.g., daily output also starts daily files unless new_file_freq_units is set).
• Errors out on a missing stream or field, on an added stream without fields, and when combined with a diag_table (or diag_table.unresolved) in SourceMods. All instances in a multi-instance run must have the same diag_table lines.
• Documented in the user_nl_mom template.

user_nl_mom and MOM_override:

• Accept full-path parameters, e.g., KPP%N_SMOOTH = 2.
• Error out when a parameter is set outside the block where MOM_input has it, which MOM6 silently ignores.
• Validate block structure (mismatched or extra closes, nested blocks, empty names) and recognize comments as MOM6 does (!, //, /* */).
• Strip whitespace left by inline comments, which produced #override instead of (UNCHANGED).
• Fix the TypeError on duplicate parameters (supersedes Fix _read_mom input err msg #350).
• Write MOM_override in one place, with one header.

Other fixes:

• Templates see MOM6 parameter values as MOM6 reads them: quoted strings no longer crash diag_table generation (e.g., WAVE_METHOD = "SURFACE_BANDS"), and T/.true. are recognized (USE_MARBL_TRACERS = T used to skip marbl_in).
• input.nml: skip variables whose guards are all false instead of writing None.
• The parser moved to a CIME-free param_file_reader.py, and tests/doctests.py runs again.

No answer changes for existing configurations.

Testing: ran buildnml for ~1140 variants (12 compsets/grids including MARBL, WW3 and B; all OCN_DIAG_MODE/TEST/sections combinations; run types; multi-instance; SourceMods; testmods; user_nl_mom edge cases) and compared the generated input files against main: they differ only where intended. aux_mom is currently being run.

Co-authored by claude.

[manishvenu]

@AidanJanney, this PR might be of interest to you!