New "unite" function/command

[standage]

This update introduces new functionality for "mating" two genotypes to create a simulated "offspring" genotype. At the Python level, this is invoked by calling the microhapulator.genotype.Genotype.unite function, and at the command-line level this is invoked by calling mhpl8r unite on two genotype files. Closes #42.

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

[codecov-io]

Codecov Report

Merging #47 into master will decrease coverage by 0.71%.
The diff coverage is 64.7%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage   98.19%   97.47%   -0.72%     
==========================================
  Files          24       24              
  Lines         775      792      +17     
  Branches      145      149       +4     
==========================================
+ Hits          761      772      +11     
- Misses          6       10       +4     
- Partials        8       10       +2

Impacted Files	Coverage Δ
microhapulator/genotype.py	97.24% <64.7%> (-2.76%)	⬇️

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