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
if you actually want to double check, check `shape  == (1, 3)`, however the current code already asserts that before calling the function so it's not necessary.