CbmBuildEventsQA: Added histograms to display percentage of correct digis per...

@se.gorbunov @f.uhlig @v.friese

Added two histograms to the CbmBuildEventsQA class to visualize the event-wise distribution of the percentage of correct digis and percentage of found digis. In order to follow the conventions of the other QA classes, a QA folder was created in the output tree and a histogram sub-folder within it, which contains the histograms.

Right now only the distributions for the full system are shown. Additional histograms for each subsystem can easily be added. Also, canvases can be added outside of the histogram folder.

Following the conventions of the other QA classes, a DeInit() function was created, which is called inside Init(). This was requested by @se.gorbunov.

The histograms span the range 0 to 100.1 instead of 0 to 100 because root seems to place edge values in the upper bin. Here this is unfortunate because the value "100 percent" occurs frequently and would not be displayed if 100 was the upper limit. I did not manage to find out how to change this behavior in root. Perhaps someone knows.

(side remark @v.friese: I was working on the requested unpacking/eventbuilding macro but lustre went offline and has not been back up, so I could not continue and did this instead)