Make Chemkit mandatory for now

The USE_* options are not ready yet, and Chemkit is required for
MongoChem at this stage.

Change-Id: Idf3f30b43103732044b9e3dab4bedb5fe827f9d5
Merge branch 'submodule-updates'

Change-Id: I54eb5f031754721ab78c90c9a0237af20fadc988