[WIP] save full signature in SBT leaves

[luizirber]

Fixes #198

• 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-io]

Codecov Report

Merging #366 into master will increase coverage by 0.06%.
The diff coverage is 94.06%.

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
+ Coverage   90.75%   90.82%   +0.06%     
==========================================
  Files          33       33              
  Lines        5010     5091      +81     
  Branches       36       36              
==========================================
+ Hits         4547     4624      +77     
- Misses        463      467       +4

Impacted Files	Coverage Δ
sourmash/search.py	93.02% <100%> (+0.1%)	⬆️
sourmash/sbt.py	85.62% <100%> (+0.02%)	⬆️
tests/test_sbt.py	98.94% <100%> (+0.09%)	⬆️
sourmash/sourmash_args.py	93.05% <86.66%> (-2.26%)	⬇️
sourmash/commands.py	91.91% <92.3%> (+0.33%)	⬆️
sourmash/sbtmh.py	88.38% <94.28%> (+1.52%)	⬆️

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[luizirber]

I'm revisiting this and some comments.

I'm getting the tree ksize from the root node (I'm initializing the BFs with a real ksize instead of using 1), but the best solution seems to be to keep both scaled and ksize as a property of the tree (NOT the root node!), and maybe even as a list. Why?

• once we have a tree structure, we can reuse it for other ksizes (but need to recalculate the internal nodes)
• it's a good way of knowing what other trees we can build with the same leaves (instead of having to go thru all the leaves and accumulating what ksizes and scaled values are available)

there is still a bunch of TODO here, so don't merge!