Enforce const correctness

[max-veit]

This is a bit of a nag really, but const correctness has been an issue ever since the start; it's never been easy with our complex network of heavy custom types, not all of which have the correct permutations of const/non-const methods implemented. Nevertheless, we should keep this fairly high on our priorities list for maintenance and cleanup, as inconsistencies caused by non-observation of const correctness tend to propagate insidiously throughout the entire codebase.