[plotter] group subplots via test names

[floweisshardt]

the image below could e.g. be be grouped via

• r1 and r2
• c0, c1 and c2 and split into subplots (new columns)

grouping should be an (optinal) parameter for plot.py

rosrun atf_plotter plot.py plot-benchmark -g r1,r2 /tmp/atf_test_app*/results_txt/atf_results.bag
rosrun atf_plotter plot.py plot-benchmark -g c0,c1,c2 /tmp/atf_test_app*/results_txt/atf_results.bag