message: Fix bug in linear constraint.
  Constraint is w*x = a, but the code had w*x-a=a
diff: @@ -149,8 +149,8 @@ public:
  int ierr = MPI_Allreduce(&loc_wtx, &wtx, 1, MPI_DOUBLE, MPI_SUM, comm_);
  MFEM_ASSERT(ierr==MPI_SUCCESS, "MPI_Allreduce failed with error" << ierr);
  #endif
  - //w' * x - a
  - cons[0] = wtx - a_;
  + //Constraint is w_ * x = a_, a_ is provided in get_cons_info
  + cons[0] = wtx;
  }
  return true;
  };
  