simplification of syntax and semantics

...e/tudresden/inf/st/pnml/base/constants/PnmlConstants.java

src/main/jastadd/base/Navigation.jrag

src/main/jastadd/base/clauses/expressions.jadd

src/main/jastadd/base/distribution/DistributedPN.jadd

src/main/jastadd/base/io/IoPN.jadd

src/main/jastadd/base/io/IoPN.relast

src/main/jastadd/base/parsing/PnmlSignalParser.jadd

src/main/jastadd/base/parsing/ToolSpecificsParser.jadd

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

src/main/resources/nets/TestNet2.pnml

src/main/resources/nets/TestNet3.pnml