GEOS-Chem submod update: Merge PR #1976 (Round state vector to int)

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

6fa3f574d Merge PR #1976 (Add fix for CH4 state vector for inversions)

This adds a bug fix for analytical inversions done with the IMI.

Signed-off-by: Bob Yantosca <[email protected]>from file to nearest integer