Simplify Embedding of novel species

[Yanay1]

This PR simplifies the process of embedding novel species not seen in the training set by adding a csv config file where they can be specified (after running through the creation notebook).

[yhr91]

Improvements for later:

• species_to_pe dictionary has to be manually emptied except for the species for which we have protein embeddings
• adata_path file path parser throws an error if file path doesn't contain './'