Dynamic cutoff calculation for genotyping

[standage]

The previously implemented automatic cutoff calculation works well for genotyping loci with high effective coverage (few discarded reads). But it has failed in a few cases where loci have a low effective coverage (microhaps with a large extent, resulting in many reads that do not span the entire microhap and thus must be discarded). In these cases, a fixed threshold performs better.

Rather than choosing the automatic approach or the fixed threshold approach, this update introduces a new dynamic approach that chooses the automatic or fixed threshold at each locus based on effective coverage.

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• [x] Changes are clearly described above
• Any relevant issue threads are referenced in the description
• [x] Any new features are tested (see docs/DEVEL.md for details)
• [x] Substantial changes are documented in CHANGELOG.md (see https://keepachangelog.com/en/1.0.0/)

[codecov-io]

Codecov Report

Merging #51 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
+ Coverage   97.18%   97.18%   +<.01%     
==========================================
  Files          26       26              
  Lines         817      818       +1     
  Branches      151      151              
==========================================
+ Hits          794      795       +1     
  Misses         11       11              
  Partials       12       12

Impacted Files	Coverage Δ
microhapulator/cli/type.py	100% <ø> (ø)	:arrow_up:
microhapulator/type.py	97.1% <100%> (ø)	:arrow_up:
microhapulator/genotype.py	96.42% <100%> (+0.01%)	:arrow_up:

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