Add some example ModelJoin files

assets/model_join/multiple_joins.modeljoin

+
+natural join imdb.Film with library.VideoCassette as jointarget.Movie {
+  keep attributes imdb.Film.year
+  keep attributes library.AudioVisualItem.minutesLength
+}
+
+natural join imdb.Film with library.Author as jointarget.Director {
+  keep attributes imdb.Film.title
+  keep attributes library.Author.name
+}

assets/model_join/theta_join.modeljoin

+theta join imdb.Film with library.Movie where imdb.Film.title = library.Movie.name as jointarget.Movie {
+  keep attributes imdb.Film.title
+  keep attributes library.Author.name
+}