Change parallelGateways, add intermediateThrowEvents, and more
This PR contains the improvements discussed after the paper review and fixes some problems. The main changes are:
- Add DataObjectReference to the palette
- Allow multiple inputs and outputs to ParallelGateway
- Fix problem of value computation in SequenceFlows output from XOR
- Add IntermediateThrowEvents
- Change, if a messageFlow is presented, show an error on -Propagate conditions- and before doing the verification
- Fix problem with relative connections and start/end events