In ReactionSystems, add the shortcut applyRS to apply a reaction system.
Define BehaviourGraph and set up some tools to build it.
Add breadOnEmptySet.
Add singletons to Properties.
Add array to the list of dependencies in brsim.cabal.
Replace breakOnEmptySet with assignComponents.

The new function can be used to retrieve several components of the same
time and is thus more general than breakOnEmptySet.
Move the functionality of calculating the connected components out of assignComponents.
Make assignComponent slightly easier to read.
Slightly improve Properties by listening to hlint.
Add intersectionKind and IntersectionKind.
Swap the arguments of intersectionKind.
Add componentIntersectionKind.
Add conservedInGraph.
Make listConservedSets use conservedInGraph.
Export BehaviourGraph from properties.
Add the function sources.
Add subgraph.
Add descendants.
Add sourceSetsDAG.
Borrow (><).