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
If this is primarily for reading Polhemus files, and Polhemus uses "%", and `write_dig_points()` uses "%", why not make "%" the default?