Define a flexible API to map between different currencies:
- map between different codes within the same namespace
- map between different namespaces
- think of grouped or leading currency instances
- think of mapping the relations between current currencies in a country and former historic ones:
- e.g. in Italy ITL was the valid currency before introduction of the EUR.
Since mapping can be implemented by several realizations active at the same time, a mechanism must be defined, how different Spi implementations can be used in parallel and transparently in the accessor implementation.