extend gene IDs, symbols, descriptions with further species

[carsten-jahn]

Using the Biomart client, we could add data for drosophila, macaca fascicularis and maybe more. This could be done by invoking the get_gene_mappings for these Tax IDs in addition, so that data for these species is added to omics_gene and omics_base just like the other species.

In case species aren't covered by Biomart and we want an "on the fly" definition of any gene IDs, symbols and gene descriptions from the gene annotation dataframe of an imported h5ad file, the import process needs to be extended. Currently, it would just raise any error message for unknown species or unknown genes.