[MRG] fix thresholds in distance_utils

[bluegenes]

The distance thresholds in the original distance utils equations were creating some issues -- this PR only skips distance estimation if jaccard/containment are actually 0 or 1.

ref: mahmudhera/phylogenetic-tree-using-fracminhash#2

[codecov]

Codecov Report

Merging #2060 (304b44f) into latest (3827367) will increase coverage by 7.45%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #2060      +/-   ##
==========================================
+ Coverage   84.28%   91.74%   +7.45%     
==========================================
  Files         130       99      -31     
  Lines       15256    10976    -4280     
  Branches     2155     2155              
==========================================
- Hits        12859    10070    -2789     
+ Misses       2098      607    -1491     
  Partials      299      299              

Flag	Coverage Δ
python	91.74% <100.00%> (ø)
rust	?

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Impacted Files	Coverage Δ
src/sourmash/distance_utils.py	99.39% <100.00%> (ø)
src/core/src/sketch/hyperloglog/mod.rs
src/core/src/errors.rs
src/core/src/sketch/minhash.rs
src/core/src/index/sbt/mhbt.rs
src/core/src/cmd.rs
src/core/src/ffi/index/mod.rs
src/core/src/from.rs
src/core/src/index/mod.rs
src/core/src/ffi/cmd/compute.rs
... and 22 more

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[bluegenes]

ready for review @ctb