GEOS-Chem submod update: Merge PR #2078 (Fix PBL level calculations)

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

a1359ddc1 Merge PR #2078 (Fix PBL level calculations)

This PR now uses local scale height and level thickness to compute
the PBL model top level.  This should be more accurate than the previous
algorithm, which used a globally-fixed scale height.

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