Split SBT into a submodule

[luizirber]

SBT is growing, and is now spread in three different places:

• sourmash_lib.sbt: the standard SBT implementation, using bloom filters for internal nodes and leaves.
• sourmash_lib.sbtmh: the MinHash extensions, with a leaf node for MinHashes and a search function.
• sourmash_lib.sbt_storage: Different storage backends for SBTs

The proposal is to create a new sbt submodule in sourmash, and move the separate files into a new structure:

• sourmash_lib.sbt, also containing possible subcommand options for SBTs
• sourmash_lib.sbt.mh
• sourmash_lib.sbt.storage

[phiweger]

This would be neat. Especially if the API allowed easy experimentation with arbitrary sequences, e.g. DNA, signatures etc.

[ctb]

-0 complicates source tree, '_' was good enough for Python 1.0 why do we need packages within packages? :) but no strong objections.

[ctb]

note that the command line API is heading in the other direction with #240 - 'sbt_search' -> 'search', etc.

[ctb]

Yes, please.