Closing the 0.9-series

Better constructors for Cascades needed by the upcoming Covering Lemma implementation.
Cascades now have a partially implemented List interface. This was needed to be able use them in HashMaps. BREAKING CHANGE: as a consequence, the ^ operation is not registered any more for OnCoordinates.
Skeleton visualisation: non-image singletons visually separated to an additional level, but not contributing to depth.
Direct depth calculation - height computation removed. Some decomposition may change as the loose components are pushed upwards.
MaxChainOfEssentialDependency added.