Fix event based digitization and reconstruction for the MVD detector

Flush the internal buffer after each event when running in event based mode. This creates digis for all channels even if the dead time of the channel is not yet reached. All digis are written into the current timeslice. Flushing the internal buffer after the last event when running in time based mode to avoid losing data.

The current implementation creates for ~99.3% of the MvdPoints corresponding MvdHits. The resulting residuals can be seen in the attached plots.

mvd_hit_error_x.pdf mvd_hit_error_y.pdf

refs #3072, #3066, #3088

Superseded by !1557 (merged)