Sharing indices

[luizirber]

• make it bagit-compatible, similar plans in https://github.com/dib-lab/sourmash/issues/991
• set the fetch.txt file with the URLs for all the signatures, but don't include the full data (it would be more than 2 TB...).
  • It would be pretty cool to have IPFS addresses in fetch.txt...
• By 'indices' here I mean SBT (without internal nodes, tree structure only) or a Linear index. LCA can be computed locally, tho.
• Check if building a taxinfo for SRA datasets is viable, and include in the index.