[MRG] switch sbt.zip over to default SBT storage

[ctb]

Makes .sbt.zip the default output of sourmash index.

Fixes #972

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1174 into latest will increase coverage by 9.55%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           latest    #1174      +/-   ##
==========================================
+ Coverage   83.33%   92.89%   +9.55%     
==========================================
  Files         103       76      -27     
  Lines        9609     5992    -3617     
==========================================
- Hits         8008     5566    -2442     
+ Misses       1601      426    -1175     

Flag	Coverage Δ
rusttests	?

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Impacted Files	Coverage Δ
sourmash/sbt.py	89.00% <95.65%> (+0.10%)	⬆️
sourmash/sbt_storage.py	91.52% <0.00%> (-0.57%)	⬇️
src/core/src/errors.rs
src/core/tests/test.rs
src/core/src/index/bigsi.rs
src/core/src/ffi/signature.rs
src/core/src/encodings.rs
src/core/tests/minhash.rs
src/core/src/ffi/mod.rs
src/core/src/index/mod.rs
... and 19 more

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[luizirber]

There is something going on with this PR. The IPFSStorage test is failing (and I think the correct fix is changing the tests to save to tree.sbt.json, instead of just tree, which gets converted to tree.sbt.zip) , but... it is taking more than 3x as long to run tests. That's... not acceptable =] (false alarm, it was the IPFS tests that were taking a long time to run)