Allow each test case to have its own (and working) slider.

src/lib/components/situations/SituationEdit.svelte

src/lib/components/test_cases/RolePersonsEdit.svelte

src/lib/components/test_cases/TestCaseEdit.svelte

src/lib/components/test_cases/TestCaseView.svelte

src/lib/components/test_cases/TestCasesPane.svelte

src/lib/components/variables/VariableInput.svelte

src/lib/components/variables/VariableView.svelte

src/lib/situations.ts

src/routes/__layout.svelte

src/routes/simulation.svelte