Generator: re-made some methods constants.
Generator: re-made some methods constants.

`const` had been removed from them while working on a different implementation, but now it's fine: we can put `const` back.

nickerso/libcellml,cellml/libcellml,hsorby/libcellml,cellml/libcellml,nickerso/libcellml,cellml/libcellml,hsorby/libcellml,cellml/libcellml,nickerso/libcellml,hsorby/libcellml,nickerso/libcellml,hsorby/libcellml