Fixes for Gaudi::Functional not using GaudiAlg

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

• explicitly include Gaudi/Accumulators.h
• use GaudiAlgorithm explicitly where needed
• adapt for the change of header file for PyAlg

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