Add cluster markers and heatmap to scRNASEQ

[ilan-gold]

In the h5ad-to-arrow, we actually have much more data than we output. We could ouptut as well:

• Cluster assignments form leiden clustering
• Heatmap of cell x expression
• Intermediate PCA/KNN visualization

This will need to be discussed as to what is visualized and what is not. For now the cluster assignments are a must.

[ngehlenborg]

We should aim to get the Leiden clusters in as soon as possible. The heatmap would be useful, too, but I think for large data sets it might get prohibitively large. It is worth testing it though.

@mruffalo would need to comment on how useful intermediate results from PCA/kNN are.