Add ExpiMap integration method

[nictru]

Description of feature

Builds up on scVI and provides better interpretability

[gregothebyteknight]

I am close to completing the Expimap integration method. However, I still have a question.
Expimap requires a reference model in GMT format. As I understand it, there is no such test data in the GitHub repository. How should I run tests and treat GMT data?

[nictru]

Hey, I did not think it through that far. If I remember correctly, this encodes the pathways the model uses to construct its latent space. So probably most users would be fine with using an existing database, but some users might want to override and use custom paths. I think it is fair to ignore the custom paths for now, and focus on a single existing database.

I think these GMT files are quite small (below 10MB?), so the easiest way will be to add it to the assets directory in the pipeline repo and access it from there. A bit like the gene lists that the people working on the cell cycle tasks have added. Make sure to add proper documentation on how the file was obtained.