GEOS-Chem submod update: Merge PR #1951 and PR #1917

This commit informs the GCClassic superproject about the following
commits that were pushed to the GitHub geoschem/geos-chem repository:

e696799c2 Merge PR #1917 (Update GEPA CH4 emissions to Gridded GHGI v2)
9afd21830 Merge PR #1951 (Update MPI usage when using CESM)

This brings code updates for CESM, plus an emission inventory update
for the carbon/CH4/tagCH4 simulations into the GEOS-Chem 14.2.1
development stream.

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>