Add MultiwaySystem properties

[maxitg]

Requires #335.
To get identical results as MultiwaySystem, requires #345, #349.

The problem

We should have support for all MultiwaySystem properties in the local multiway system.

Subtasks

Feel free to create separate issues for each of these properties, and add more properties to this list. Some of these might have local, more efficient versions, which should be implemented first:

• StatesGraph StatesGraph #348;
• EvolutionEventsGraph;
• EvolutionCausalGraph;
• CausalGraphInstances;
• BranchPairsList;
• BranchialGraph;
• StateWeights.