clean up marbl_tracer_index_type and marbl_tracer_metadata_type

[mnlevy1981]

I don't think we need tracer_module_name element in marbl_tracer_metadata_type or any of the marbl_tracer_count_type objects in marbl_tracer_index_type (or the marbl_tracer_count_type derived type itself).

The only use for tracer_module_name is letting the users know which tracers came from which modules, which doesn't seem like a necessary feature. The indices in the marbl_tracer_count_type were used to set specific fluxes and tendencies to zero before computing them, but we set the full arrays to zero at the beginning of the respective _compute() functions so those calls were redundant. There is also some information about each tracer module in the log:

base biotic tracers tracer module contains 32 tracers; indices are 1 to 32
abio tracer module contains 2 tracers; indices are 47 to 48
ciso tracer module contains 14 tracers; indices are 33 to 46

but do we really need to provide this information at runtime?