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
```
chs = self.info['chs']
pos = np.array([chs[k]['loc'][:3] for k in picks])
```
avoids an array copy.