examples.diffusion.mesh1D No-flux - steady-state doesn't always give zero

[agoncalvescoel]

When I run mesh1D.py , the other parts of the script run fine, but the part on the figure below gives me different results

The blue line does not appear at all

And when it uns the validation of numerix, gives me False

[guyer]

So, the code is working properly, but the explanation in the text is not completely correct. Those lines are only tested with PySparse, which does return zero everywhere. Other solvers can return infinity or NaN. Regardless, the solution isn't right, which is the point of the illustration. I'll work on improving the explanation.