FEAT: implement `FormFactor` expression class #505

Job	Run time
pytest / Determine job matrix	5s
style / Determine skipped pre-commit hooks	9s
doc / linkcheck / Check external links	1m 13s
doc / nb / Execute and build documentation	1m 2s
pytest / Run unit tests (3.10, ubuntu-22.04)	1m 34s
pytest / Run unit tests (3.11, ubuntu-22.04)	1m 33s
pytest / Run unit tests (3.12, ubuntu-22.04)	1m 40s
pytest / Run unit tests (3.7, ubuntu-22.04)	2m 11s
pytest / Run unit tests (3.8, ubuntu-22.04)	1m 36s
pytest / Run unit tests (3.9, ubuntu-22.04, ampform)	4m 27s
pytest / Run unit tests (3.9, macos-12)	2m 40s
style / Run taplo through pre-commit	19s
style / Run local pre-commit hooks	1m 39s
doc / Upload to GitHub Pages	0s
style / Push changes	0s
	20m 8s

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