Terminology: container versus element tree versus element

[kleihan]

The v1.0 spec uses all 3 terms and the difference is unclear. This should be made consistent.

Examples:

An internal <codeSet> is a container for <code> elements. 
It is a reference to a `<field>` element within the `<fields>` container.
In the schema, a `<field>` element is contained by parent element `<fields>`.
The `<messages>` element contains any number of child `<message>` elements.
The `<actors>` element tree provides a place to store such external state information.
The `<categories>` element tree is used to associate FIX elements to business areas.
The `<sections>` element tree names higher level business processes.