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
can you update the example http://martinos.org/mne/dev/auto_examples/plot_ssp_projs_topomaps.html#example-plot-ssp-projs-topomaps-py

so we just do:

evoked. plot_projs_topomap()

?