Parse Headers and Return Raw Packet Data Only with Empty XtcePacketDefinition

[medley56]

Context

In some use cases, people will want to parse CCSDS headers, returning the binary data directly and not worrying about the user data definitions. They will not have an XTCE document, nor do they need one for this use case.

Implementation Plan

• Allow instantiation of XtcePacketDefinition without an XTCE document.
• When we parse a packet stream for headers only, we should get back the raw packet data for the packet along with the parsed header. This may already be the case.
• Add a test for parsing packets without a packet definition.