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
Not sure what you mean. What I meant is that if we have a variable `"nasion"`, why do you create a string `"nasion"` and submit it to a function alongside with the variable when you could just submit the `nasion` variable  etc.?