ENH add_dig_points, SetChannelMixin, reorganization

This is WIP.

Adds function apply_dig_points to apply hpi, cardinal, and dig points to the Info.

Remove unnecessary import functions for np.loadtxt.

Needs test. Not ready for review
I would allow ch_type=None is which case all types ch_type if present are plotted in different figures. Functions with default arguments are so convenient.