XTCE12 — ByteOrderList in XTCE 1.1 needs to be improved

ByteOrderList is an element that describes the byte order of encoded packet info – mnemonics. Each mnemonic may have a different byte order. ByteOrderList is currently optional. The problems with it are:
1) Not specifying it means the item is most significant byte first. But many adopters of XTCE fail to realize this initially at least producing many bad XTCE files.
2) When specifying it, each "significance" is given a numerical value. The exact mechanism or meaning of this is not explained in the annotation. Results are a lack of commonality by XTCE file authors.
3) There are 2 popular byte orders which cover many cases: "big" and "little" endian (or most/least significant byte first). Explicitly specifying these produces a lot of XML but a clearer XTCE file.
Suggested fix: make the item required or provide an explicit default setting. This possibly means moving it to an attribute. Make the defaults the common settings, provide a bail out for the less common byte orders. Improve the annotation.

Create "byteOrder" attribute

Create "byteOrder" attribute similar to existing "bitOrder" attribute. Legal values for this attribute should include:

• Common byte orderings (similar to BitOrderType)
  • "mostSignificantByteFirst" (default)
  • "leastSignificantByteFirst"
• Arbitrary byte orderings (for backwards-compatibility with v1.1)