FEAT: implement `AmplitudeModel.masses` and `.invariants` #258

Job	Run time
style / Determine skipped pre-commit hooks	12s
pytest / Determine job matrix	5s
doc / linkcheck / Check external links	56s
doc / nb / Execute and build documentation	1m 23s
pytest / Run unit tests (3.10, ubuntu-22.04)	18s
pytest / Run unit tests (3.11, ubuntu-22.04)	19s
pytest / Run unit tests (3.12, ubuntu-22.04)	22s
pytest / Run unit tests (3.8, ubuntu-22.04)	21s
pytest / Run unit tests (3.9, ubuntu-22.04, ampform_dpd)	24s
pytest / Run unit tests (3.9, macos-12)	35s
style / Run local pre-commit hooks	1m 14s
style / Run taplo through pre-commit	20s
doc / Upload to GitHub Pages	0s
style / Push changes	0s
	6m 29s

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