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
When we started adding EEG electrode locations we added them as ELP points, hence there could be more than 8. I have't used it in a while though, has this changed in the meantime?