Plot systems

This new `plot-systems` command is now very flexible although messily written. It should be played around with and we should see what's good for a TAC summary in paper or web site form.

TODO:
* [ ] Documentation
* [ ] String substitution for display
* [ ] sort by name should be case-insensitive
* [x] Systems and measures on one plot (e.g. heat map)
* [x] Precision and recall on row/col plot
* [x] custom figure size? (less important now that interactive fine-tuning of figures is possible)
* [ ] ignore no-variance measures in CompareMeasures