refactor the sbt_gather code to be sane(r)

[ctb]

note, PR into #135.

also fixes a bug in find_best when threshold is set to 0.

• add test for 0 threshold

---

• 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?

[brooksph]

This works!

running sourmash subcommand: sbt_gather

loaded query: all.fna... (k=21, DNA)
query signature has max_hash: 1844674407370955
found: 1.00 0.31 NC_000853.1 Thermotoga maritima MSB8 chromosome, complete genome
found: 0.90 0.28 NC_011978.1 Thermotoga neapolitana DSM 4359, complete genome
found: 1.00 0.26 NC_002163.1 Campylobacter jejuni subsp. jejuni NCTC 11168 = ATCC 700819 chromosome, complete genome
found: 0.48 0.15 NC_009486.1 Thermotoga petrophila RKU-1, complete genome
found 4 matches total
the recovered matches hit 0.0% of the query

Composition:
1.00 NC_002163.1 Campylobacter jejuni subsp. jejuni NCTC 11168 = ATCC 700819 chromosome, complete genome
1.00 NC_000853.1 Thermotoga maritima MSB8 chromosome, complete genome
1.00 NC_011978.1 Thermotoga neapolitana DSM 4359, complete genome
1.00 NC_009486.1 Thermotoga petrophila RKU-1, complete genome