layout_kamada_kawai() maxiter should scale with graph size

[szhorvat]

layout_kamada_kawai() uses maxiter=1000 by default. The recommendation in the C core docs is:

 * \param maxiter The maximum number of iterations to perform. A reasonable
 *        default value is at least ten (or more) times the number of
 *        vertices.

The Mathematica and R interfaces both use 50*vcount.

The Python interface should also use a maxiter value that at least scales with the vertex count.

[ntamas]

Fixed this, see above; tests are failing on my machine, but it seems to be unrelated (they failed even without this change). I'll continue investigating tomorrow.