Put the message decoders in sorted order.
Fix concatenating segments.

The size in the message header includes the message header itself, so
this needs to be subtracted.
Check that the message parsers use all the data they get.

This is currently failing for Message 13.
Add support for NEXRAD message 18 (RDA Adap. Data.)

This relies on parsing a text file that was extracted and hand-massaged
from the PDF of the ICD. The script takes the text and generates the
proper code.