Update seedpixel and cluster timestamp

This MR contains the changes discussed in #109 (closed). Closes #109 (closed).

• Cluster::getSeedPixel():
  • before: returns pixel with largest charge, if charge=zero: random pixel
  • now: returns pixel with largest charge, fallback if charge=zero: earliest pixel
• Clustering4D: set pixel timestamp:
  • before: use earlist pixel (which was not equal the seed pixel)
  • now: use pixel with largest charge, if not available use earliest pixel + configurable (maybe remove parameter)