Support caller-provided RHS of linear equality constraint.

[ambrad]

For efficiency, CEDR computes the RHS of the linear equality constraint as part of the same single reduction. But some users will want to provide the RHS. Support this option.

[ambrad]

A current workaround is to provide its value as Qm_prev in the call to set_Qm on the root rank, with all other ranks passing Qm_prev as 0. But the interface should formalize this in some nice way.