Change ReserveDetDescForEvent to use LHCb::Algorithm

Make ReserveDetDescForEvent inherit from LHCb::Algorithm::Transformer instead of directly using Gaudi::Functional::Transformer.

This is a backward compatible change needed to prepare for gaudi/Gaudi!1444 (merged).

Part of a set of independent MRs: Rec!3395 (merged) Allen!1193 (merged) Analysis!975 (merged) Boole!466 (merged) !4082 (merged)