Add the ability to have trailing empty members on NamedStruct.

This allows them to be filled in later.
Add some more converter types.
Decode the RDA channel in the message header.
Some clean-up in prep before adding message 1.
Put the RDA alarms in the struct.
Add method for checking sizes at the end of messages.
Add support for decoding message 5, VCP info.

This also allows fully decoding RDA adaptation data.