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
I was trying to remove the projs before the evoked was generated. this is not working for some reason. in the example, it loads the projs separately and generates the layout from the evoked. this is not working when the projs are added to the evoked. the existing four are still there.