added support for external functions

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

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

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

src/main/java/de/tlc4b/analysis/DefinitionsAnalyser.java

src/main/java/de/tlc4b/analysis/MachineContext.java

src/main/java/de/tlc4b/analysis/StandardMadules.java

src/main/java/de/tlc4b/analysis/Typechecker.java

src/main/java/de/tlc4b/analysis/UsedStandardModules.java

.../tlc4b/analysis/transformation/DefinitionsEliminator.java

src/main/java/de/tlc4b/exceptions/TranslationException.java

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

src/main/java/de/tlc4b/tla/TLAModule.java

src/main/resources/standardModules/ExternalFunctions.tla

src/test/java/de/tlc4b/prettyprint/ClausesTest.java

src/test/resources/basics/ExternalFunctionsTest.mch