Add DECLARE_CONVERTER forward-compatible macros
As a step towards dropping the DECLARE_FACTORY macros (gaudi/Gaudi!420 (merged)), introduce DECLARE_CONVERTER, which will allow us to start migrating our code and be both compatible with the master of Gaudi and our v30r1 ATLAS branch.
@leggett could you tag this and switch master to it? I will then push changes to the converters in athena, which should then also fix the build failures against Gaudi master.