Fix function signatures

This fix contains four commits:

• replace final override by override. One is redundant and while to use of final is not wrong, the choice was to make all signatures override for consistency. final just prevents further inheritance of the member function.
• apply modernize-use-override to add override where it is missing
• apply modernize-use-equals-default to make empty ctor/dtor declaration default.
• apply readability-make-member-function-const to make members function const where applicable. This does not work for (pure) virtual functions and overrides as clang-tidy cannot check if functions can be made const in the derived classes.