suzerain: #3021 make enthalpy cold/hot friendly

Adjusts the enthalpy thickness definition to behave correctly
(i.e. positively) regardless of cold vs hot wall conditions.
Remains TBD for enthlpy Re and thickness in the presence of
baseflow.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43128 0bd85f0b-c151-43b9-a1e9-27231104551f