Move run selection out of straxen, pax conversion fixes
Created by: JelleAalbers
This accompanies https://github.com/AxFoundation/strax/pull/155. It moves the abstract run selection code (i.e. as much as possible, but nothing MongoDB-specific) out of straxen and into strax.
This also fixes the pax->strax conversion so that it also works on triggerless XENON1T data. We also add a new samples_per_record
option to specify how to break pulses into records, which is convenient to change for LED (1000 or 1400 samples/pulse depending on the mode) and noise (100000 samples/pulse) data.