FIX: use squared `FormFactor` in `EnergyDependentWidth` #515

Job	Run time
pytest / Determine job matrix	7s
style / Determine skipped pre-commit hooks	8s
doc / nb / Execute and build documentation	1m 20s
doc / linkcheck / Check external links	55s
pytest / Run unit tests (3.10, ubuntu-22.04)	1m 41s
pytest / Run unit tests (3.11, ubuntu-22.04)	1m 40s
pytest / Run unit tests (3.12, ubuntu-22.04)	1m 42s
pytest / Run unit tests (3.7, ubuntu-22.04)	2m 20s
pytest / Run unit tests (3.8, ubuntu-22.04)	1m 43s
pytest / Run unit tests (3.9, ubuntu-22.04, ampform)	4m 30s
pytest / Run unit tests (3.9, macos-12)	2m 2s
style / Run taplo through pre-commit	22s
style / Run local pre-commit hooks	1m 31s
doc / Upload to GitHub Pages	0s
style / Push changes	0s
	20m 1s

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