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
you could use np.asarray with dtype=np.float in set_channel_positions to be able to pass lists. It would make tests easier to read.