Speed-up layout computation.

[thocevar]

Problem:
Computation of node positions with the Fruchterman-Reingold force-directed layout method presented a bottleneck in the Network Explorer widget.

Solution:

• Use a sample/subset of nodes to estimate repulsive forces acting on each node. Experiments showed similar resulting layouts (with a significantly lower runtime).
• Introduce callback_rate parameter to detach the number of callbacks from the number of iterations of the layout algorithm.

Result:
Continuous drawing of calculated node positions is a new bottleneck. It can be mitigated with the use of callback_rate.

[kernc]

Got:

AttributeError: 'ProgressBar' object has no attribute 'set'

[kernc]

I don't notice much difference but understand it should be there. It works.

I made the adaptation of callback_rate=.25 as replotting even iterations results in smoother animation.