Define rotation axis center for Radon transforms with inverses

When studying a sinogram, the position along the sinogram corresponding to the center of rotation must be known (tomography reconstruction algorithms also need this information, of course). This PR intends to do the following:

1. Document the definitions of rotation axis position used in skimage
2. To make 1. simpler, redefine the rotation axis position for some cases. This redefinition makes it much simpler to explain skimage's behavior, and it also simplifies the code.
3. Perform some cleanup which is made possible by point 2. A highlight is to redefine the boundary of the reconstruction circle to be included in the circle.

I think all of the above has been attended to, so this is ready for merge, but the changes need review first, of course!

When writing this code, I needed to decide on a definition of the kind "Where is the center of an array?". My definition is:
```python
center = tuple([s // 2 for s in a.shape])      # a is some array
```
I hope this does not contradict any other definition in use in skimage.

@stefanv I think this will be my last PR on cleanup of the radon/iradon code. I am quite happy with the code now. However, there still might be more tomography-related features coming. In particular, I hope @emmanuelle will have time to push her total variation reconstruction code into skimage :)