ENH: improve evoked.plot_image with channel layouts

Evoked images (heatmap, channels on y axis) are often plotted grouped by hemisphere, e.g.

This is IMO nicer to interpret than the current MNE function. I have homegrown routines, is anyone interested in me trying to integrate them in MNE eventually? API would probably be adding a grouping= keyword to evoked.plot_image.

[mmagnuski]

@jona-sassenhagen Looks good!

I often like masking the insignificant (or otherwise uninteresting) values with transparency. I actually have lots of matlab code for this, one example:

The channels are grouped differently here (frontal, central, posterior and each group has channels from left (top) to right (bottom)) - and it would be nice if both ways were possible here (in mne). If you want to take a look at the matlab code it is here and here for example (but it’s messy). BTW masking transparently with colors not from the current colormap is a nightmare in matlab (or maybe just used to be) but a pleasure in matplotlib 😃

[jona-sassenhagen]

I am thinking about doing an example on this later. With the merging of #5010, probably don’t need main code support. Closing.