suzerain: add map conserved/pressure

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42574 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: rename largo_baseflow_{derivative,dx}

Want a largo_baseflow_dy to go with the existing data,
and nearly everything else is ambiguous or clunky.
Basewards compatible code for fields from before r42574

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42575 0bd85f0b-c151-43b9-a1e9-27231104551f