Add support for continuation packets

[greglucas]

Context

Continuation packets occur in CCSDS Packets when the data fields are too long for a single packet delivery, or if logically a team wants to break up the packets into smaller chunks. The packet data should be concatenated together when we encounter continuation packets in the XTCE parsing logic.

Implementation Plan

Add checks for the sequence flags within the packet generation code and do the concatenation for users.