Handling extensible records.

src/main/java/de/tla2b/analysis/TypeChecker.java

...ain/java/de/tla2b/exceptions/NotImplementedException.java

src/main/java/de/tla2b/types/StructOrFunction.java

src/main/java/de/tla2b/types/StructType.java

src/main/java/de/tla2bAst/BAstCreator.java

src/test/java/de/tla2b/expression/ComplexExpressionTest.java

...test/java/de/tla2b/prettyprintb/ExtensibleRecordTest.java

src/test/java/de/tla2b/prettyprintb/RecordTest.java