Refactor models

All the models in this repo (and others) are now available in ReachabilityModels.jl see https://github.com/JuliaReach/ReachabilityModels.jl/issues/11  :tada:  

The next step would be to cleanup the `/models` folders in this repo, and write benchmark scripts with specific groups of models to be tested.

cc  @SebastianGuadalupe 