TPTools+AthenaPoolCnvSvc: Add const interfaces for calling TP converters.
Introduce TPConverterConstBase. This works like TPConverterBase except that the methods called on the derived class are const, and take arguments by reference instead of by pointer.
Also add TPCnvStdVectorConst.
Add aliases: T_AthenaPoolTPCnvConstBase = TPConverterConstBase T_AthenaPoolTPCnvStdVectorConst = TPCnvStdVectorConst