Describe the problem

The connectivity circle visualization function currently does not allow much control over the colorbar. I think many users might want to set a label for the colorbar and control the tick locations to produce figures closer to publication quality. It appears somewhat difficult to change these and find the colorbar instance after the figure is created.

Describe your solution

It's very simple of course. I can make a PR adding a few parameters to the plotting function here and in mne.viz.circle if this is something others consider valuable. Perhaps something like colorbar_ticks and colorbar_label.