[WIP] Extend ModelJoin writer to write to FS

build.sbt

project/plugins.sbt

...l_join/representation/grammar/CompoundKeepExpression.java

..._join/representation/grammar/KeepReferenceExpression.java

...el_join/representation/grammar/KeepSubTypeExpression.java

..._join/representation/grammar/KeepSuperTypeExpression.java

..._join/representation/writer/FileBasedModelJoinWriter.java

...el_join/representation/writer/ModelJoinWriterFactory.java

...oin/representation/writer/StringBasedModelJoinWriter.java

...ation/writer/FileBasedModelJoinWriterAcceptanceTests.java

...ion/writer/StringBasedModelJoinWriterAcceptanceTests.java

...l_sync/model_join/representation/writer/TestedModels.java