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
It's now a little bit asymmetric to have `write_dig_points` but no `read_dig_points`. I would at least specify in a "Notes" section how to read the files that this function writes. Or what about adding a simple `read_dig_points()`? you could do the `pts.shape[1] != 3` check in the read function (I assume they're always 3 dimensional).