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
right, so speaking of which I see in `brain vision.py` (line 619) the code below. Shouldn't that be done with make_dig() now?

```
        ch_locs = _get_elp_locs(elp_fname, elp_names)
        info['dig'] = [{'r': ch_locs['nasion'],
                        'ident': FIFF.FIFFV_POINT_NASION,
                        'kind': FIFF.FIFFV_POINT_CARDINAL,
                        'coord_frame':  FIFF.FIFFV_COORD_HEAD},
                       {'r': ch_locs['lpa'], 'ident': FIFF.FIFFV_POINT_LPA,
                        'kind': FIFF.FIFFV_POINT_CARDINAL,
                        'coord_frame': FIFF.FIFFV_COORD_HEAD},
                       {'r': ch_locs['rpa'], 'ident': FIFF.FIFFV_POINT_RPA,
                        'kind': FIFF.FIFFV_POINT_CARDINAL,
                        'coord_frame': FIFF.FIFFV_COORD_HEAD}]
```