Test out PerovStats skeletonisation in TopoStats

[tobyallwood]

The skeletonisation code in the PerovStats project was originally based on TopoStats's version, however got cut down to be shorter while still giving the same results.

It would be helpful if this could be added to TopoStats, replacing the longer version and increasing efficiency and maintainability.

[ns-rse]

By chance I noticed some activity on a pull request I made on the up-stream skan package three years ago which I never
finished off. The package performs skeletonisation of objects.

The pull request Add support for iterative pruning based on user-specified discard conditions by ns-rse · Pull Request
#214 · jni/skan saw some improvements from the developer of the package and makes
it possible to prioritise pruning of different types of branches (if I understood correctly from my brief skimming of
the commits).

It might be worth looking at including and using skan for pruning again as it would reduce the amount of code within
TopoStats that needs maintaining if it fulfills the requirements, if it doesn't contributing upstream benefits everyone
(and the developer was very open to contributions and supportive when I worked with him three years ago).