ENH: initial draft of PR2 fetching code

[mikerobeson]

This is just a heads up on some simple code to fetch the PR2 reference SSU data.

To do:

• Add test code.
• Add taxonomy rank labels?
  • Versions can have different levels of curated taxonomy.
    • Version 5.0.0 has 9 taxonomic ranks:
      • domain | supergroup | division | subdivision | class | order | family | genus | species
    • Earlier versions have 8 taxonomic ranks, etc...
      • kingdom | supergroup | division | class | order | family | genus | species
• Add code to fetch even older versions?
  • Versions 4.13 and earlier have the SSU data as separate 16S and 18S rRNA files. Can handle this by downloading and merging these files.