suzerain: #2979 prep repurpose enforcer

The IsothermalPAPTEnforcer scaffolding will be repurposed for
both isothermal lower/upper as well as isothermal lower/nrbc upper
conditions.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43832 0bd85f0b-c151-43b9-a1e9-27231104551f