ENH: Add repr for inverse

This PR might betray my knowledge of forward/inverse stuff. Do we need anything else in the `repr`?
> As far as I understand compute_orient_prior and compute_depth_prior would give you the matrix given the depth/loose parameters, but there is no function to go the other way round. I could try to write one if that's high on the priority list.

yes it was the idea to revert the depth + loose from these matrices