Add Unitary Group

[HiperMaximus]

I think adding the unitary group U(n) and special unitary SU(n) would be interesting as they are 'bigger' than O(n) and SO(n) (and also connected which might be interesting), and many physical models are invariant under the unitary group. I'm uncertain if it would necessitate adding support for complex numbers.

[maxxxzdn]

I think at this point, escnn is focused on all subgroups of the Euclidean group E(n), so adding support for unitary groups might be a heavy lift. Perhaps one can develop uscnn for that matter :)