Metric-based optimization

[PhilipMathieu]

NetworkX has a number of built-in layout algorithms. Many of these mirror the options included with GraphViz. However, none of them appear to be written in a way that would permit providing an external cost function. The idea behind this enhancement would be to write an optimizer, based on the existing algorithms or other literature, that can operate based on the cost returned by a metric from the MetricSuite.