an error with PDE - pickling error

[vietvo89]

Hi Pablormier

I ran your example with parallel process:
from yabox import PDE
PDE(lambda x: sum(x2), [(-10, 10)]).solve()**

Then I got an error as below:
PicklingError: Can't pickle <function at 0x7f4ef807e268>: attribute lookup on main failed