Enforce a minimum thickness of 0.5*Angstrom in the mixed_layer_restrat
routines.  The streamfunctions in these routines attempt to limit the
thicknesses to exceed Angstrom, but they can be less than this due to roundoff.
The new limit prevents thicknesses from becoming negative when Angstrom is set
to 0, but should not change any answers for test cases with positive values of
Angstrom.  Also added some comments describing variables and their units and
simplified the OMP directives.

  Also corrected error messages in MOM_diabatic_aux.F90 to identify the file or
module where these messages come from, and modified an error message in
applyTracerBoundaryFluxesInOut so that it is written if the localized fault does
not happen to occur on the root PE.

  All answers in the existing MOM6-examples regression suite are bitwise
identical.