Add different colors to report plots

[standage]

This PR updates the API to support plotting histograms in the mhpl8r pipe report with different colors. Closes #138.

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• [x] Changes are clearly described above
• [x] Any relevant issue threads are referenced in the description
• 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/)