Remove scikit-learn dependency

[danielhollas]

scikit-learn is only used for PCA to align molecular axes in SmilesWidget. Pulling such a heavy dependency just for this is an overkill. I am not sure if it is even still needed, and I am sure there must be less heavy-weight to do it.

https://stats.stackexchange.com/questions/235882/pca-in-numpy-and-sklearn-produces-different-results

Past PRs for context:
#160
#164