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
the reason for it is that it would be repeated code outside of the module. you would still have to apply the trans twice regardless, for the `elp` and then for the `hsp`. i still feel that a dig constructor would be the most effective way for generality across systems.