C2MS — Have required attributes in the messages

Suggest favoring having required attributes in the messages rather than encouraging applications to hardcode looking for the nth element of the subject, as different existing GMSEC-based systems already have different numbers of tokens in the subject.

• While these items may exist in the body of a message, it has been found that some existing GMSEC consuming apps are extracting from subjects

Add verbiage re: required attributes in messages; move CONSTELLATION-ID, and the 2 SAT-IDs to Required Fields

In Figure 8-3 (Message Header), move CONSTELLATION-ID, and the 2 SAT-IDs to Required Fields.
Edit paragraph 8.1.1 to change "Some of the elements of the C2MS subject name can be made up from fields in the C2MS Message Header." to "All of the elements of the C2MS subject name can be made up from required fields in the C2MS Message Header."