addCLtoLowerCS enables to select only one stratum at a time

[rix133]

RDBEScore::addCLtoLowerCS does not support pooling natively — its inner subset_dt uses ==, which silently misbehaves on a multi-value stratum. For support, the RDBEScore would nee a change dt[get(field) == criteria[[field]], ] to dt[get(field) %in% criteria[[field]], ] in that inner subset_dt