Function for creating Affine and Projective transforms

It would be useful to have (fairly straightforward) functions Affine2::new and Affine3::new (or similar names), that take an (n x (n+1)) matrix, and return an affine transform (by appending the row (0, 0, ... 0, 1)). This prevents the necessity to call from_matrix_unchecked each time one wants an affine transform to be created.