Haplotype call plots

[standage]

This PR adds a new API function for generating haplotype call plots: a .png is created for each marker, showing read counts of the alleles above the detection threshold, as well as the analytical threshold. This PR also integrates this API call into the mhpl8r pipe workflow.

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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 the development manual for details)
• [x] CLI documentation (see docs/cli.md) and Python API documentation (see microhapulator/api.py) are up-to-date and in sync
• [x] Substantial changes are documented in CHANGELOG.md (see https://keepachangelog.com/en/1.0.0/)