More inverses.

We can now go to and from the Luo spaces and CIECAM02.
Probably the interface of this module and also the interface of srgb.py should be harmonized with what's going on in ciecam02.py. 
I'd suggest making this
```
RGB = r.rand((10,) * num_dims + (3,))
XYZ = np.asarray(sRGB_to_XYZ(RGB)) * 100
```
which is shorter and supports arbitrary `num_dims`. Then we can test 3-dim inputs too :-)