GEOS-Chem submod update: 0.5x0.625 CH4 simulations use 300s/600s time…

…steps

This commit informs the GCHP superproject about the following
    GEOS-Chem submod update: 0.5x0.625 CH4 simulations use 300s/600s timesteps

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

abca1a549 Bug fix: 0.5 x 0.625 nested CH4 simulations will use 300/600 sec timesteps

This fixes an issue that was discovered during GC Classic integration
tests, where an error trap was triggered due to the 300s/600s timesteps
for CH4 nested-grid simulations.

Signed-off-by: Bob Yantosca <[email protected]>