[feature] Message printed in terminal when concentration becomes

negative.

cellfile.py

@@ -1016,6 +1016,10 @@ class CellFile(object):
                                 BC_values_i[flux_indices_i][:, input_time_i] \
                                 / RdX_flux_i
                 u[i] = u_i
+                for j, elt in enumerate(u[i]):
+                    if elt < 0:
+                        print("Negative concentration ({0}) in cell "
+                              "{1}".format(elt, j+1))
 
             current_time += time_window
             # update cell identities