Corrected SubgraphConnector to handle subgraphs without supergraphs:
-if there is no current supergraph, we do not try to connect, preventing NullPointerException
Merge branch 'develop' into PiMM-algorithm_checker
Created a PiGraph checker, which checks Refinements and Fifos of a given PiGraph.
It can be launched by a pop-up menu: right-click on a .pi file > PREESM > Check algorithm
or by a workflow task which id is org.ietr.preesm.pimm.checker.task.PiMMAlgorithmCheckerTask and inputs and outputs are a PiGraph