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
Well so if there is actually another location where this would be used, then I would create a simple `construct_dig` function that basically duplicates the old code.

I find the new `add_dig_points` strange because it's basically two functions (the if/else statement that encompasses almost the whole function) which has to e called twice to run both subfunctions, and then it reduplicates a lot of functionality (applying a trans multiple times even though it has already been applied before even calling it for the first time). I think a simple thing would do like 

```
def construct_dig(nasion, lpa, rpa, head_shape_points):
    <old code here>
```