IC: Add Manning/Strickler equation for IC approximation.

src/Model/Geometry/Profile.py

@@ -345,3 +345,9 @@ class Profile(object):
     # Abstract method, must be implemented for in non abstract class
     def get_station(self):
         raise NotImplementedMethodeError(self, self.get_station)
+
+    # Computation method
+
+    # Abstract method for width approximation
+    def width_approximation(self):
+        raise NotImplementedMethodeError(self, self.width_approximation)