Use a macro to generate the repeating code.

The resulting code should be identical.  This change is needed because
to implement the JSON conversion in Base we will need more big
repetitive switches code.
Implement Message::stringToDataType().

And convert dataTypeToString to use a common type table.
Also return coding type for physical types.
Dummy Message payload iterator API.
MessageJsonConverter::messageToJson implementation.

Untested.  MessageJsonConverter is the successor of
MessageSerialConverter (wasn't implemented anyway).