Restructure specification Chapter 3.9 - Message Structures

[kleihan]

This chapter contains the specification for components, groups, and messages and the rules for member presence. Other element types (scenarios, datatypes, code sets, fields) have their own level 2 chapters.

The proposal is to elevate the elevate the level 3 chapters inside 3.9 to level 2 and move "Member presence" to the end of these, i.e.

3.9 Components
3.10 Repeating groups
3.11 Messages
3.12 Member presence
3.13 Expressions (changes from 3.10)
...