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
ah ok, then it's just slightly unexpected that write_dig_points writes pickles that read_dig_points does not read... hm... maybe you could keep that functionality secret :)