ENH: introduce sorting of scatter dots according to their color

Introduce option for plots where the plotting order of the scatter dots is sorted according to their color, i.e. if we have -1 and 1 as possible colors, then all the dots with color=-1 is plottet before those with color=1. It is useful for doing band structure plots with spin-orbit coupling where the coloring is given by <n|s_z|n>.

Default for sortcolors is False, should it rather be True?