Make the set_x_affine functions public

[nilgoyette]

We realized while working on a terrible ANTs affine bug that those functions should be public, so that the programmer has more options than this method:

pub fn set_affine<T>(&mut self, affine: &Matrix4<T>)
where
    T: RealField,
    T: SubsetOf<f64>,
    T: ToPrimitive,
{
    // Set affine into sform with default code.
    self.set_sform(affine, XForm::AlignedAnat);

    // Make qform 'unknown'.
    self.set_qform(affine, XForm::Unknown);
}

This is our (Imeka) way of doing things, but it may not be everyone else's way.

[Enet4]

This is fine by me. The documentation on set_sform is a bit thin, but other than that, they are both documented and it should be relatively clear what is meant.

👍