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
Just noticed this, if you remove `write_mrk()` you also need to update the GUI so that it does not allow you to save as pickle!