Implemented 'partial invariant evaluation'

src/main/java/de/tlc4b/TLC4B.java

src/main/java/de/tlc4b/TLC4BGlobals.java

src/main/java/de/tlc4b/Translator.java

...sis/unchangedvariables/InvariantPreservationAnalysis.java

...analysis/unchangedvariables/UnchangedVariablesFinder.java

src/main/java/de/tlc4b/prettyprint/TLAPrinter.java