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
If there is no benefit for users that justifies a commitment to maintaining the function for backwards compatibility then it should be private. There might be a use case for users that need to fix their files, like

```
points = read_dig_points(…)
points = … # fix my file
write_dig_points(points)
```=