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
For example, the way it handles the trans it basically forces you to call it twice, and each time it regenerates the ras_to_neuromag_trans. It achieves the same thing with at least twice the lines of code