Implement a network-appropriate plot() method for mixingmatrix. #95
Description
Heat maps are often used to visualise mixing matrices, but the default method for table objects uses mosaic plots. For networks, it's also meaningful to adjust the counts for network composition. For example, number of nodes with a particular attribute value could be mapped to the height or the width of the corresponding cells on the heat map, and the edge count and/or density and/or residual relative to independence can be mapped to the cell's colour.