Adding function s() for performing expression substitution in a string
Adding getEdgeDensity, isWeighted and summarise Graph methods
Using Graph#isWeighted
Tweaks to graph plot method, including ability to use vertex names
Improvements to Graph levelplot method, to bring it into line with plot.Graph
Renaming newGraphWithVertices() to inducedSubgraph(); allowing vertices to be specified by name
Renaming newGraphWithEdgeWeightThreshold() to thresholdEdges(); adding ability to binarise the final graph
Splitting calculateMetricsForGraph() into separate functions