add p/t net execution (both in-place and by creating new markings)

src/main/jastadd/Navigation.jrag

src/main/jastadd/marking/Execution.jadd

src/main/jastadd/marking/Marking.jrag

src/main/jastadd/marking/Printing.jrag

src/main/java/de/tudresden/inf/st/pnml/Main.java