Slugs

Add an interface to the [`slugs`](https://github.com/LTLMoP/slugs) synthesis tool. Derived from a5d1196 by @vraman. Note that this is based on #95, so that has to be merged before this. For now it requires the `slugs` version from branch `python` [here](https://github.com/johnyf/slugs/tree/python), in particular installing the `slugs` python package under `tools`.

The input to `slugs` admits a slight superset of GR(1), where next operators are allowed inside liveness clauses. Variables can appear on both sides of comparators.