Re-add src/tracer/MOM_generic_tracer.F90

In mom-ocean#1664, src/tracer/MOM_generic_tracer.F90 was moved to config_src/external/GFDL_ocean_BGC/MOM_generic_tracer.F90. This was done to simplify COBALT v3 development. However, having MOM_generic_tracer.F90 in the generic_tracers codebase doesnt work when compiling generic_tracers as a library, so we maintain MOM_generic_tracer.F90 in the MOM6 codebase.

Author: dougiesquire