"--resume" now evaluates the files already produced

For both data_for_charting.tsvand taxon_to_mmap_to_orthologs .json:

• if the --resume parameter was used and the file is found, KEGGCharter won't generate it again.
• else, KEGGCharter will again generate the data, and overwrite the file if it exists.

Also, an important fix

On retrieving kegg taxa prefixes - checks with type(taxa) == str now, instead of taxa != np.nan.