Use Leiden clustering instead of Louvain

[nsohail19]

Default setting in FindClusters() is Louvain, however best practice is to use Leiden. Update materials and resultant cortex subclustering using the leiden clusters

[sjhosui]

This is not urgent so let's leave it as is for now. I'd like to dig deeper into the differences we observe in the two clustering algorithms before we make the transition. Thanks!