Refactor genotype simulation and sequencing code

[standage]

This update makes sweeping changes to the genotype simulation and sequencing code.

• The sim module no longer performs sequencing and focuses entirely on haplotype simulation.
• The seq module now handles simulated Illumina sequencing of both simple (single contributor) and mixture (multi contributor) samples.
• The mix module merges simple genotypes into a simulated mixture sample.
• The mixture module has been dropped, and its functionality is covered by the more granular sim, mix, and seq modules.

This update also replaced all references to microhapulator.cli.parse_args() with microhapulator.cli.get_parser().parse_args(). The former is used to configure runtime logging, which for some reason causes issues in a testing environment.

Closes #44.

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• [x] Changes are clearly described above
• [x] 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 #45 into master will increase coverage by 0.66%. The diff coverage is 97.23%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   97.69%   98.35%   +0.66%     
==========================================
  Files          22       24       +2     
  Lines         780      789       +9     
  Branches      145      146       +1     
==========================================
+ Hits          762      776      +14     
+ Misses          9        5       -4     
+ Partials        9        8       -1

Impacted Files	Coverage Δ
microhapulator/cli/__init__.py	94.28% <100%> (+0.16%)	:arrow_up:
microhapulator/panel.py	98.06% <100%> (+1.31%)	:arrow_up:
microhapulator/cli/sim.py	100% <100%> (ø)	:arrow_up:
microhapulator/cli/mix.py	100% <100%> (ø)
microhapulator/cli/seq.py	100% <100%> (ø)
microhapulator/mix.py	100% <100%> (ø)
microhapulator/sim.py	100% <100%> (+6.09%)	:arrow_up:
microhapulator/__init__.py	97.82% <100%> (+0.04%)	:arrow_up:
microhapulator/genotype.py	100% <100%> (ø)	:arrow_up:
microhapulator/seq.py	93.82% <93.82%> (ø)
... and 5 more

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