Added import warning to python hypervolume

Author: fmder

deap/tools/_hypervolume/pyhv.py

@@ -23,11 +23,14 @@
 #    License along with DEAP. If not, see <http://www.gnu.org/licenses/>.
 
 from math import log, floor
-
 import random
+import warnings
 
 import numpy
 
+warnings.simplefilter("once", ImportWarning)
+warnings.warn("Falling back to the python version of hypervolume "
+    "module. Expect this to be very slow.", ImportWarning)
 
 def hypervolume(pointset, ref):
     """Compute the absolute hypervolume of a *pointset* according to the