build a streamlined Genbank/RefSeq based on GTDB 25k genomes

[ctb]

ref #4 and #7, I think providing a small database using the GTDB 25k genomes, but with NCBI names/taxonomies instead, would be quite useful to many people.

the logic is that:

• NCBI taxonomy is a mess, but people like it and are used to it;
• since GTDB 25k is a nice low-redundancy collection of genomes, they're good to match against;
• so we could provide just those genomes, but with NCBI taxonomy instead of GTDB taxonomy.

I guess we'd want to make sure the names are NCBI names where possible, and we'd want to provide a lineages CSV with it.

see also sourmash-bio/sourmash#969

[ctb]

closing in favor of our modern database build processes, will document elsewhere.