Scenario: Add delete scenario command and use undo command.

src/Model/Scenarios.py

@@ -68,3 +68,10 @@ class Scenarios(PamhyrModelDict):
                 self._dict
             )
         )
+
+    def delete(self, key):
+        el = self._dict.get(key)
+        if el is None:
+            return
+
+        el.set_as_deleted()