Consider adding explicit conversion methods between the various transformations.

Currently, the user has to call `na::convert` and `na::try_convert` to convert from one transformation type to another. This approach seems difficult to find out about (cf. https://github.com/rustsim/nalgebra/issues/364#issuecomment-422208316) so we might want to add methods with explicit names as well like `Isometry3::to_affine()`.