Add peak center times to event and peak basics

Created by: JelleAalbers

This:

• Computes the weighted center time of peaks, i.e. sum_t t * w[t] where w[t] is the peak's sum waveform at time t and adds it to peak_basics;
• Adds the center time of the main and secondary S1 to event_basics
• Uses the center instead of the start time of peaks to compute the drift time

Thanks to @DonNabla for pointing out the importance of this for doing robust 83mKr analysis.

We could consider computing still other peak times, such as the time of the sum waveform, or the time of median area.