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
can't you do a local relative import? I don't like this import from the mne namespace that implies to use the installed mne. I fear it could be error prone.