Allow virtual flux correction based on ocean salt restoring/correction

[dougiesquire]

This PR implements changes that allow stf adjustments based on any salt restoring/correction applied by the ocean model. There are two changes in this PR:

• A generic_tracer::generic_tracer_update_from_coupler routine has been added. This receives the salt flux restoring/correction applied by the ocean and passes it on to the generic tracer's update_from_coupler routine.
• An optional input flux_virtual has been added to the generic_tracer_utils::g_tracer_add routine. If true, this ensures that the stf field is allocated on the tracer being added.

Unfortunately, changes are needed to the calling ocean model in order to utilize the changes in this PR:

• In MOM5, the existing call to generic_tracer::generic_tracer_coupler_get is replaced by a call to generic_tracer::generic_tracer_coupler_accumulate and then generic_tracer::generic_tracer_update_from_coupler is called as part of ocean_sbc_mod::flux_adjust. See here for the changes.
• In MOM6, the salt flux added is accumulated and generic_tracer::generic_tracer_update_from_coupler is called in MOM_generic_tracer::MOM_generic_tracer_column_physics. See here for the changes.

Merging this PR but not changing the ocean model will mean no change from current behaviour.

Closes #32