GEOS-Chem submod update: Merge 14.2.0-alpha.19, PR #1935, PR #1931

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

4192e3263 Post-merge fix: Update arg list of stub fullchem_SetStateHet routine
fee0cb249 Merge PR #1935 (Fix comments in unitconv_mod.F90)
d4ddcd5df Merge PR # 1931 (Add area-dependency warning in GCHP HISTORY.rc files)
6aa9dc592 Merge 14.2.0-alpha.19 into dev/14.2.1 branch

We have now merged 14.2.0-alpha.19 into dev/14.2.1 (which was based off
14.2.0-alpha.15), plus PR #1935 and PR #1931 (which only update comments).

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