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
Ok thanks! So this makes me think, instead of exposing `mne.io.meas_info.make_dig_points`, wouldn't it be useful to expose a `Raw`/`Epochs`/... or `Info` method `.add_dig_polhemus(point_file, point_names, hsp_file)`? That way anyone using Polhemus data for coregistration could add them to whatever format they are collecting their data in.