BC: Make node association unique (#25).

src/Model/BoundaryCondition/BoundaryConditionList.py

@@ -89,3 +89,11 @@ class BoundaryConditionList(PamhyrModelListWithTab):
 
     def copy(self):
         return copy(self)
+
+    def get_assoc_to_node(self, tab, node):
+        return next(
+            filter(
+                lambda i: i.node is node,
+                self._tabs[tab]
+            )
+        )