Deprecate free-functions that are also available as methods.

[sebcrozet]

Currently, several functions are defined at the crate root. Those functions exist from the time where nalgebra used generics extensively, requiring the user to import lots of traits to use a given method.

Now that those functions are available as methods without needing any trait import, they should be deprecated. Those deprecation will prevent the user being confused by the fact there exist several ways of doing a single thing.