Add computation time statistics
Add custom axis plotting

For large numbers of steps the time axis was unreadable due to too
long, overlapping labels. Now, when the number of steps is over 8000,
labels are printed as 'XK', e.g. 10000 -> 10K.
Use plain Agg backend for non-GUI plots

Using pyplot FSM API resulted in attempt to use Qt-specific backend for
plots intended for saving to file. This requires issuing Qt drawing
commands from non-GUI thread, which is non-desirable, and sometimes
leads to outright crash. Non-GUI plots code now uses pure object
matplotlib API with Agg backend, without pyplot.
Add time step computation duration plot
Add option to switch off GUI