Move MOM_generic_tracer.F90 to ocean_BGC

[theresa-cordero]

Move MOM_generic_tracer out of MOM6 to allow changes to the interface between MOM6 and generic tracer packages to be tailored to each group of generic tracer packages.

This should not change answers.

[theresa-cordero]

This PR should be tested with PR 21 to dev/cefi MOM6.

[yichengt900]

@theresa-morrison, thank you for opening this PR. I love the idea of moving MOM_generic_tracer.F90 to the ocean_BGC repository, as it provides additional flexibility for future development. I only have a minor concern: the MOM_generic_tracer.F90 in this PR does not pass the current MOM6 format check. To address this, I’ve opened another PR (#108), which is nearly identical to this one but includes a minor cleanup of MOM_generic_tracer.F90 to ensure it complies with the MOM6 format check. This change could be useful if we decide to replace the existing version of MOM_generic_tracer.F90 in the current GFDL MOM6 repository. Feel free to review and share your thoughts.

[yichengt900]

@theresa-morrison, I will go ahead and close this PR since #108 has been merged.