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
I am not sure this will work. You need to check with a for loop if self contains "eeg", "grad", "mag"

unless find_layout does this for you.