Rename group or person situation to population.

src/lib/components/situations/RolePersonsEdit.svelte

src/lib/components/situations/SituationEdit.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