Base time

[uramirez8707]

Description
Currently, base_time, base_year, base_month, base_day, base_hour, base_minute, base_second are PUBLIC module variables, that can be changed at any point. They are set in diag_table_mod when the diag_table is read.

Note: The base_time IS used in model code, in places like:
https://github.com/NOAA-GFDL/atmos_phys/blob/80d0a701d9772e23f44e6163cfd22ec0d762d0d9/atmos_param/radiation/radiative_gases/radiative_gases.F90#L2258

This PR:

1. changes these to PRIVATE and adds getters for each variable
2. get_base_time is made public by diag_manager to reproduce previous behavior:
3. Adds a set_base_time routine which sets these variables as it was done in diag_table_mod
4. A private logical module variable, base_time_set, prevents users from changing them
5. The set_base_time routine is then called in diag_table_mod when reading the diag_table and in fms_diag_yaml when reading the yaml file
6. A test was added to test this routine.

How Has This Been Tested?
CI Passes which includes a test for this
c96L33_am4p0 experiment still reproduces with this branch (as is) intel prod-openmp, debug

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes