First draft of generalized WCS standard.

WIP.

At this point, I'm deliberately trying to avoid some of the rigidity of FITS WCS.  All FITS WCS have a `CDELT` offset and `PC` matrix, for example, as the "core" concept.  But I think that's limiting.  What if you want to correct on the focal plane first, for example?
In that case, I think we would just have a transformation type for each of the arithmetic operations we need.  (We have summed and scale already, but those could just as easily be called add and multiply).  

But this does sort of bring your two questions together.  The proposed description of "summed" here included dimension mapping, so summed has three properties: "transforms" (for the list of transforms), "inputs" and "outputs" (to map the dimensions).  So it's a little verbose for what amounts to simple addition:

```
!wcs/summed
transforms:
    - first_transform
    - second_transform
inputs:
    x: ra
    y: dec
outputs:
    ra: x
    dec: y
```

That's pretty verbose.  If you take the dimension mapping out of summed itself, you don't have to have the extra level of the tree different properties (assuming the dimensions are already in the form that you want them).  i.e.:

```
!wcs/summed
    - first_transform
    - second_transform
```

You'd still have to do something if the dimensions aren't how you want them.