Added a `to_owned` method to `MatRef` and `MatMut` (#21)

* Added a `to_owned` method to `MatRef` and `MatMut`
Added a `to_owned` method to `MatRef` and `MatMut` to make a `Mat` from
non-owning variant. The method is an inherent method and not tied to the
`ToOwned` trait as that would conflict with it's blanket implementation.
For further infos see https://stackoverflow.com/questions/72105604/implement-toowned-for-user-defined-types