restructured simulator code to allow for other types of controllable components

villas/controller/component.py

villas/controller/components/simulator.py

villas/controller/components/simulators/__init__.py

villas/controller/components/simulators/dpsim.py

villas/controller/components/simulators/dummy.py

villas/controller/components/simulators/generic.py

villas/controller/components/simulators/rscad.py

villas/controller/components/simulators/rtlab.py