improve RawBank::BankType usage
Required such that Allen can eventually use RawBankTypes without include RawBank.h - see Allen!2143 (merged)
Note: this MR is required by LHCb!5323 (merged), but it can be applied 'by itself' -- i.e. it must be applied either prior or together with LHCb!5323 (merged).
Edited by Gerhard Raven