MARBL_tools gets tracer count wrong for non-CESM PFT setups

[mnlevy1981]

The way MARBL_tools:MARBL_settings_class determines tracer count assumes the user is running with PFT_defaults = 'CESM2'. This is fine for running out of the box, but causes problems when PFT_defaults = 'user-specified' and will also be problematic in the future when we try to implement tiered complexities.

The problem stems from this block of code in the YAML:

_tracer_cnt : &TRACER_CNT
   default : 32
   increments :
      lvariable_PtoC = .false. : -3
      ciso_on = .true. : 14

To start with, we need to allow multiple defaults depending on PFT_defaults so the above values are only captured if PFT_defaults = 'CESM2'. We also need to offer the user a way to change in via input file (such as letting a CESM user put _tracer_cnt = 54 in user_nl_marbl); that will require hard-coding a check for _tracer_cnt in input_dict before processing the tracer_dependent category.

Following the above recomendation, the default should be _tracer_cnt = -1 if PFT_defaults = 'user-specified' and then we can abort if the user does not change the tracer count to a positive value.

[mnlevy1981]

Fixed in marbl0.25.0