GEOS-Chem submod update: Merge PR #2211 (Initialize PBL variables)

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

4279f837e Further updates in Compute_Pbl_Height for clarity & performance
a3c1e5f54 Merge PR #2211 (Initialize PBL variables)

This fixes the "bad sum" error encountered in PR #2078, by making sure
that variables are zeroed each time that PBL quantities are computed.

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