[WIP] Provide first draft of ModelJoin writer

build.sbt

...el_sync/model_join/representation/core/AttributePath.java

...ync/model_join/representation/grammar/JoinExpression.java

...model_sync/model_join/representation/util/Functional.java

...odel_sync/model_join/representation/util/JoinFactory.java

...sync/model_join/representation/util/ModelJoinBuilder.java

...ct/model_sync/model_join/representation/util/Nothing.java

...oject/model_sync/model_join/representation/util/Pair.java

...ync/model_join/representation/writer/ModelJoinWriter.java

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

...n/java/org/rosi_project/model_sync/util/NeedsCleanup.java

...ion/writer/StringBasedModelJoinWriterAcceptanceTests.java