Robustness to zero-gain channels, add Peaks options
Created by: JelleAalbers
This goes along with https://github.com/AxFoundation/strax/pull/187 to fix https://github.com/AxFoundation/strax/issues/177:
- Remove hits from zero-gain channels before clustering.
- Remove to_pe option from PeakBasics and PeakPositions. It's not needed anymore since area_per_channel will now get units of PE.
- Make more of the clustering options in Peaks configurable.