Remove unused variable in solver mapping routines
Allow constraining solver DOFs to be equal

Tracker item 2204

This allows mapping multiple equation set DOFs to a single solver DOF
but is currently quite fragile and also needs to be moved to within the
boundary conditions structure and allow for more general linear DOF
constraints in the future.
Merge remote-tracking branch 'remotes/adamreeve/solver_dof_constraints' into solver_dof_constraints
Updating solver dof mapping to work with interface matrices

Tracker item 2204
Merge branch 'solver_dof_constraints' into fc