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
@agramfort the examples  there are already projs that are activated when I load the evoked. same if I set the proj param to False. I can't remove the when I add the projections because they have already been applied.