Removed the colormap related helpers, we don't need since beautify.
Removed set_labels, that one is trivilal
Removed unused calc_grid
Removed plot_spectrum -- trivial
Smoother grid for the 2D interpolation of the scalp plots.
Enhanced testdata
Enhanced _subplot_scalp to ax_scalp.

This method does not create an Axes but rather uses an existing one. Also
removed a lot of stuff from this method in order to make it simple enough to
understand.

Adapted the methods using _subplot_scalp to behave correctly.

Documented it accordingly.
Adopted example script to the new ax_scalp method.