Replace Matrix types with gonum Transform

When https://github.com/gonum/gonum/issues/1797 is accepted this can be discussed and implemented. * Replace `sdf.m44` with `r3.Transform` * Replace `sdf.m33` with `r2.Transform` This should not be a breaking change for anyone who has not been using `sdf.m44` and `sdf.m33` types for anything other than the `sdf.Transform3D` and `sdf.Transform2D` functions.