Make Subroutine.argnames return lower-case names

[reuterbal]

To avoid patterns like this, and because Fortran is case-insensitive by default, we should make Subroutine.argnames return lower-case arg names .

    routine_arg_names = [arg.lower() for arg in routine.argnames]

[No action] We should make the routine.argnames property lower-case by default.

Originally posted by @reuterbal in #671 (comment)