Spherical colors

[alexmsrh]

I added two colour spaces: the barycentric colour space and spherical colour space. You can try the new features with the notebook demo_color_space.ipynb.
Main issues:

• changing color space takes time because of "display" purposes (i.e., it is not related to the computation of the densities). This happens for both the barycentric and spherical color spaces. However, reducing the resolution significantly reduces the time.
• The log-scale change in the barycentric color space is done manually (which is much slower than when done with the numpy argument norm). I could not link the selected scale directly into the ternary package used for barycentric plots. To fix this, one should probably look into ternary.colormapping.colorbar_hack
• cannot draw on the barycentric plot. I did not try to pass a mask through the ternary objects, this could be investigated.
• conversion from spherical density to spherical color space