Add operator() to PythonFunction

For simplicity we make PythonFunction callable.
In the long run, we can/should implement this
independently of the VirtualFunction interface.

dune/fufem/python/function.hh

@@ -101,6 +101,14 @@ class PythonFunction :
             callable_(x).toC(y);
         }
 
+        /**
+         * \brief Evaluate function
+         */
+        RT operator()(const DT& x) const
+        {
+            return callable_(x).template toC<RT>();
+        }
+
     protected:
 
         Python::Callable callable_;
@@ -172,6 +180,14 @@ class DifferentiablePythonFunction :
             derivative_(x).toC(y);
         }
 
+        /**
+         * \brief Evaluate function
+         */
+        RT operator()(const DT& x) const
+        {
+            return callable_(x).template toC<RT>();
+        }
+
     protected:
 
         Python::Callable value_;