Correlations: add some histograms
This MR adds some histograms to Correlations
which were very handy at the test-beam at DESY in Aug'20":
- correlationTimeOverSeedPixelRawValue: time correlation vs. cluster seed pixel
- correlationTimeOverTime_px: time correlation vs. time on pixel level
- correlationTimeOverPixelRawValue_px: time correlation vs. pixel raw value (on pixel level)