This is a trivial but useful extension to genprefixmap.py

Add a format option tsv or csv

This will be rftab-compliant (see https://github.com/ontodev/rdftab.rs#prefixes). See https://github.com/cmungall/semantic-sql/blob/main/prefixes/prefixes.csv for an example

Make sure that this option is documented in sphinx, see #446