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
that was more or less the functionality I started with. I think the function you propose could exist but it is independent of this function. it would be something that would belong to the channels module.