Wrap GenBank, FASTA, GFF3 SynBioHub download options

[cjmyers]

Add GenBank, FASTA, GFF3 SynBioHub download options.

[bertfordley]

This functionality, particularly for GFF3, would be extremely helpful. Would this be included in the next release?

[bbartley]

Although it is not currently possible to download these alternative formats directly from Synbiohub, it is possible to call a converter tool. See https://pysbol2.readthedocs.io/en/latest/getting_started.html#converting-to-and-from-other-sequence-formats