sourmash index describe (or similar)

[bluegenes]

It would be nice to have a function(maybe sourmash index describe) that reports index characteristics (scaled, ksize, moltype, sbt version, storage type?, etc).

We can currently get this info by running sourmash sig describe | head, and inferring from the signature info.

@luizirber (on slack): sourmash index describe also opens the possibility of making sourmash index into a larger subcommand (like sourmash sig), but then I think we need to move current functionality into sourmash index build or something like that