Add Sholl analysis for Skeleton object

[jni]

Supersedes #148

@kushaangupta could you please continue your work in a new branch starting from this one? To add a documentation page, you can write it in myst-nb syntax (or export a notebook to that syntax). We'll update the docs infrastructure in a different PR to make sure that gets rendered correctly in the site.

[kushaangupta]

I was thinking the following changes:

1. Since the intersection_counts length may not match shell_radii, On line 1214:

intersection_counts = np.bincount(shells, minlength=len(radii)) // 2

2. To return the computed center. I think that to not to return it is a wasted computational effort. May be it can be added as a property of Skeleton object itself?

[jni]

Thanks for the comments @kushaangupta! Both good suggestions, I've implemented them.

[jni]

Cool, this just needs a couple of tests and is ready to go in I think...!

[kushaangupta]

Oh great work! Sorry, I couldn't work on it. Got a bit confused trying to make tutorial notebook on it.

_path_distances is now not being used given the updated Sholl implementation. Should it be deleted?