Plots: rewrite BG histogram plot for 1 stream (now called hist_bg_single)
Plots: rewrite BG histogram plot for ALL streams (now called hist_bg)
Plots: functions can pass hints for plotting (dict _plot_status).

- Better default autoscaling
- Remove old hist_bg_fit function
Plots: use hint for no autoscale in timetrace and ratetrace. Also move some function.