CbmBuildEventsQa: Treatment of noise was improved.

- Noise digis are now disregarded when matching a reconstructed event to a MC event.
  (only events which are pure noise are matched to -1)
- A histogram was added which shows the fraction of correctly matched digis of reconstructed events,
  which disregards noise digis during matching.
- A histogram was added which shows the fraction of noise digis of reconstructed events.