Tests

Add `SmoothedConstantInterpolation` #327

Job	Run time
Tests (1, ubuntu-latest) / Tests	12m 6s
Tests (1, windows-latest) / Tests	14m 48s
Tests (lts, macos-latest) / Tests	8m 42s
Tests (1, macos-latest) / Tests	10m 30s
Tests (lts, ubuntu-latest) / Tests	9m 56s
Tests (lts, windows-latest) / Tests	11m 56s
Tests (pre, macos-latest) / Tests	6m 59s
Tests (pre, ubuntu-latest) / Tests	12m 26s
Tests (pre, windows-latest) / Tests	14m 0s
	1h 41m 23s

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