Add PBC support to `merge_split_MEST`

[w-k-jones]

Adding support for periodic boundary conditions in merge_split_MEST would be a nice goal during the v1.5.x release cycle. This could be done using the scikit-learn BallTree distance search approach used in feature detection. This would replace the cdist distance search approach, which can cause memory issues for calculating merges/splits for large numbers of cells.

I'm working on adding this, but may need support from @kelcyno if I run into any unexpected issues