Can't use number of pvs in selection algorithm

Trying to use the number of PVs as input in the Bs2GammaGamma selection algorithm leads to crashes in Allen.mdf_input. A loose "print debug" on a stack build led me to believe the crash happens when Allen starts processing the second slice of events. This feature was being implemented in !807 (merged), but I've removed it (a3778ce8) so the MR can be merged.

FYI @rangel