[integration/peerlab] feature: adding a reference gene correlation

[ananya-nandula]

[WIP]

[Tobiaspk]

Good PR.

Here's what should be changed before we can merge:

• Don't test if norm is a layer. Instead, ensure that inputs are raw counts and normalise the same way as anndata
• Add a parameter fill_missing_gene_correlations (or similar) to ignore errors when genes are missing in the reference
• Remove "# ADDED" and "# NEWLY ADDED" comments. Instead, add those in the PR description.

Creating some tests now to double check these changes