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 would just spell it out

```
In [1]: timeit ['hpi', 'hpi', 'hpi', 'hpi', 'hpi']
10000000 loops, best of 3: 119 ns per loop

In [2]: timeit ['hpi'] * 5
1000000 loops, best of 3: 282 ns per loop
```