Put information for initializing tracers into marbl_tracer_type

POP has an io_read_fallback_mod module that can be used to tell the model "if [tracer name] isn't available in the initial condition file, initialize it some other way"; that other way could be "read a different tracer," "read a different tracer but scale it by some factor", or "set it to a constant." (The implementation for the MARBL tracers is here.)

Given the general design principal that the GCM should have as little semantic knowledge of MARBL as possible, it makes sense for MARBL to provide the GCM information on what to do if the tracer is not available in the IC file. I think it would make the most sense to extend tracer_metadata_type:

  type, public :: marbl_tracer_metadata_type
     character(len=char_len) :: short_name
     character(len=char_len) :: long_name
     character(len=char_len) :: units
     character(len=char_len) :: tend_units
     character(len=char_len) :: flux_units
     logical                 :: lfull_depth_tavg
     character(len=char_len) :: tracer_module_name
+    character(len=char_len) :: fallback_option
+    character(len=char_len) :: fallback_tracer
+    real(r8)                :: fallback_const
+    real(r8)                :: fallback_scalef
  end type marbl_tracer_metadata_type

Where fallback_option could be 'none', 'alt_tracer', or 'const' and the other new variables are pretty self-explanatory. POP could continue to use io_read_fallback, but instead of explicitly defining the fallbacks in the linked code snippet, it would just loop through the tracer metadata and register any tracer where fallback_option is not 'none'.

marbl-ecosys / MARBL

Put information for initializing tracers into marbl_tracer_type #449