Add assertion to make_hanging_node_constraints() to check that the AffineConstraints object does not store the entire index set in parallel.

[bangerth]

As the title says. It happens too often that people don't initialize AffineConstraints with a locally relevant index set and end up with an object that stores information about every DoF (not just the locally relevant ones), running out of memory.

AffineConstraints itself cannot check this because it knows nothing about whether it is used in parallel. But we can at least check for this in make_hanging_node_constraints() which knows both the AffineConstraint object and the DoFHandler we are building constraints for.