fix all dispersive material updates to work with a universal reference, i.e. templated rvalue reference:

template
void do_material_update(YeeCell && f){ ... code ... };

Some variables are undefined if compiled WITHOUT optimization. This affects static constexpr class members which are not integral type (double, char[], etc...). Not sure what to do about this besides always compile with optimization ON

the problem is that the std::function<void(CellType &)> should be able to take either rvalue or lvalue references to CellType. However, it does not work with both, just one or the other