Sediment: Add GRA file reading.

src/Model/Results/River/River.py

@@ -98,6 +98,9 @@ class River(object):
     def reach(self, id):
         return self._reachs[id]
 
+    def has_reach(self, id):
+        return 0 <= id < len(self._reachs)
+
     def add(self, reach_id):
         reachs = self._study.river.enable_edges()