Move empty and non-existent data_table handling to data_override

[GFDL-Eric]

Is your feature request related to a problem? Please describe.
The decision of whether to call data_override_init is currently handled within the various FMScoupler coupler_main.F90 routines (simple, full, SHiELD). Since some setups don't have a data_table or a grid_spec.nc, the current implementation is the only way to prevent a "grid_spec.nc not found" crash in data_override. While the current setup is adequate as a stop-gap measure, the handling of the data_table file's existence and contents should be done within data_override itself.

Describe the solution you'd like
Remove the checks from FMSCoupler regarding data_override_init and just call the subroutines there. Within data_override, check to see if the file exists (within the read_data subroutine for the ASCII data_table, within the yaml_parser for reading the data_table.yaml). If the file does not exist, or its contents are empty, exit out of the data_override_init routine before reading in the grid_spec.nc to avoid crashes.

Describe alternatives you've considered
Keeping it as is.

Additional context
Since this involved parser changes for any yaml inputs, and sub-module that uses the yaml parser will likewise be affected when yaml parsing is implemeted in the future (diag_manager, field_manager, etc.).