Add the VSKNN_STAN algorithm

[bkersbergen]

Add the VSKNN_STAN algorithm.

The VSKNN_STAN algorithm is introduced in the paper: Empirical Analysis of Session-Based Recommendation Algorithms

VSKNN_STAN combines the ideas from STAN and VS-kNN in a single approach and adds a sequence-aware item scoring
procedure.
Its very likely that the VSKNN_STAN algorithm can leverage the VMISIndex to do its computation.

A python implementation can be found here:
https://github.com/rn5l/session-rec/blob/master/algorithms/knn/vstan.py