GEOS-Chem update: Merge PR #2617 (KPP submodule update)

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

29f6c72c4 Merge PR #2617 (KPP standalone submodule update)

This PR updates the KPP/standalone submodule hash to point to
a bug fix (write KPP standalone output with 3-digit exponents,
to avoid corrupted output).

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