Allow flavor selection in `bc.st.highly_variable_genes()`

Hi,
After Samuel Rutz's talk today at the single cell meeting, I think it would be good to incorporate the `flavor` param from scanpy's function (instead of only allowing the default 'seurat'). https://scanpy.readthedocs.io/en/stable/generated/scanpy.pp.highly_variable_genes.html

I would also suggest setting the default flavor to `seurat_v3`
Thanks,
Llucia