Add set size to heatmaps / co-occurence matrices

[LukasWallrich]

By default, the co-occurence matrices are now sorted based on the number of records, with the largest source on top (but the user can turn that off so that the ordering in the data is preserved). In addition, it might be nice to show the size of each source as in the example below? However, that is not trivial, so not something I can implement at the moment. If we want it, the best way might be to use the superheat package.