Reservoir: Allow to select only internal node.

src/View/Reservoir/Table.py

@@ -56,9 +56,22 @@ class ComboBoxDelegate(QItemDelegate):
     def createEditor(self, parent, option, index):
         self.editor = QComboBox(parent)
 
+        nodes = list(
+            map(
+                lambda n: n.name,
+                filter(
+                    lambda n: not (
+                        self._data.is_upstream_node(n)
+                        or self._data.is_downstream_node(n)
+                    ),
+                    self._data.nodes()
+                )
+            )
+        )
+
         self.editor.addItems(
             [_translate("Reservoir", "Not associated")] +
-            self._data.nodes_names()
+            nodes
         )
 
         self.editor.setCurrentText(index.data(Qt.DisplayRole))