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 feel it's too technical for what's new. Explain what it means for user. They have a get_channel_positions method for data containers.