has_heterogenous_adjoint_constraint changes

New method has_heterogenous_adjoint_constraints tests for any such
constraints.

Old method has_heterogenous_adjoint_constraint new returns the
constraint rhs Number rather than bool; this should be semantically
backward-compatible with the old system unless someone was manually
setting a zero constraint and expecting that to count as heterogenous.
Integrate regularized flux QoIs in FEMSystem