New string boundary bicreater: append

dmnverifierapi/src/main/java/de/unikoblenz/fgbks/base/utils/boundary/bicreater/BiCreaterStringAppend.java

 package de.unikoblenz.fgbks.base.utils.boundary.bicreater;
 
+import static de.unikoblenz.fgbks.base.utils.boundary.bicreater.BoundaryBiCreaterType.COMBINE;
+import static de.unikoblenz.fgbks.base.utils.boundary.checker.BoundaryCheckType.IS_IN_CONTACT;
+
+import de.unikoblenz.fgbks.base.utils.boundary.Boundary;
 import de.unikoblenz.fgbks.base.utils.boundary.impl.StringBoundary;
 import java.util.Optional;
 
@@ -22,6 +26,10 @@ public class BiCreaterStringAppend extends AbstractBoundaryBiCreater<StringBound
 
   @Override
   public Optional<StringBoundary> create(StringBoundary b1, StringBoundary b2) {
-    return Optional.empty();
+    if (b1.checkWith(IS_IN_CONTACT, b2)) {
+      return Optional.empty();
+    }
+    Optional<Boundary> b = b1.createBi(COMBINE, b2);
+    return b.map(boundary -> (StringBoundary) boundary);
   }
 }