Figured out one issue with GHAT (enhanced diffusivity), still have rel errors O(1e-4) though... next step is to track down where those large errors occur and figure out why
Verified that differences between native POP and CVMix are 1e-10 or
smaller for nonlocal term!

This commit comments out all of the diagnostic output (calls to
MNL_DEBUG), sets VDC and VVC to BLMC2, and uses GHAT2 instead of GHAT
for KPP_SRC.