Adding test for changes in kernel.pyfunc

If kernel.pyfunc has changed, the kernel needs to be recompiled. This
fixes #197

Author: erikvansebille

parcels/particleset.py

@@ -221,7 +221,7 @@ def execute(self, pyfunc=AdvectionRK4, starttime=None, endtime=None, dt=1.,
                          kernel errors.
         :param show_movie: True shows particles; name of field plots that field as background
         """
-        if self.kernel is None:
+        if self.kernel is None or self.kernel.pyfunc is not pyfunc:
             # Generate and store Kernel
             if isinstance(pyfunc, Kernel):
                 self.kernel = pyfunc