C2MS12 — At Next Major Revision: Evalutate all Required/Optional/Dependent states for consistency

Some fields and message elements that should be required are listed as optional. For example, in the Telemetry CCSDS Packet Message, ME SCID was added in 1.1, so had to me marked optional, but it should be required. APID already existed in that message as a Message Element, but not as a field, so whereas the ME is required, the field had to be made optional (for backward compatibility).

Additionally, in section 6.2.2 Format of C2MS Subject Names, there is a description that says, "If an element is required but not applicable for that mission or to that type of message, the publisher inserts “FILL” (no quotes) for that element." However, this should be cleaned up. If a field is required it should be required, if not, then it should not be required. Need a run through the messages to see how this can be improved.