add talweg in update_rk

(cherry picked from commit 674247db09)

src/Model/Geometry/Profile.py

@@ -80,6 +80,9 @@ class Profile(object):
             return self.points[0]
         elif name == "np":
             return self.points[-1]
+        elif name == "talweg":
+            zm = self.z_min()
+            return next((p for p in self.points if p.z == zm), None)
         else:
             return next((p for p in self.points if p.name == name), None)
 
@@ -88,6 +91,12 @@ class Profile(object):
             return 0
         elif name == "np":
             return len(self.points) - 1
+        elif name == "talweg":
+            zm = self.z_min()
+            return next(
+                (p for p in enumerate(self.points) if p[1].z == zm),
+                None
+            )[0]
         else:
             return next(
                 (p for p in enumerate(self.points) if p[1].name == name),