HEMCO submod update: Sync HEMCO 3.7.1 and docs/dev

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

30be8c7 Bring the docs/dev branch up to date with HEMCO 3.7.1

This update synchronizes the docs/dev and dev/3.7.1 branches to
the same commit in advance of the HEMCO 3.7.1 release.

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