Switched

This is an extensive overhaul of `transys.labeled_graphs`, switched system synthesis taking advantage of `multiprocessing` on multicore machines, (partial) debugging of `discretize` and `polytope`, rewrite of abstraction classes, separation of the `polytope` subpackage to an independent package, new partition classes that migrated to it, projecting `MealyMachine` on partitions and plotting its spatial transitions, as well as various smaller changes. Relevant issues: #74, #71, #62, #59, #57, #56 and possibly comments scattered among other issues.