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
@dengemann what would be the function in mne.viz ? it uses here plot_projs_topomap from mne.viz and there is here a mixin method.