marbl_init_tracer_metadata() passes non_living_biomass_ecosys_tracer_cnt to
marbl_init_non_autotroph_tracer_metadata(), where it is an intent out. However,
the variable is only used in the non_autotroph_tracer_metadata() [it is not
used after being returned], so I updated the interface accordingly.

Next commit will look at how to remove it altogether.

What was previously marbl_init_non_autotroph_tracer_metadata has been renamed
so that tracer is plural: marbl_init_non_autotroph_tracers_metadata; a new
marbl_init_non_autotroph_tracer_metadata routine has been introduced for
changing the metadata one tracer at a time. This removes the need for
non_living_biomass_ecosys_tracer_cnt, which was implicitly requiring that the
non-living biomass tracers had tracer indices 1 - N.

Also fixed a typo in an error traceback.

Removed a couple of un-necessary arguments from the subroutine interface, which
also let me remove an associate statement in
marbl_init_non_autotroph_tracers_metadata.