[ACTION 510] Use of TTML Version in Timed Text Toolkit (TTT)

I have completed a review of the TTT codebase and find that the following
uses of TTML specification version (as a applied to a specific document)
are used during processing:

   - [ttv] select between version specific default profiles
   - [ttv] select between TTML schemas associated with different
   specification versions (56 instances)
   - [ttv] contextualize length expressions in terms of version (in order
   to support new units) (2 instances)
   - [ttv] enable deprecation warnings (1 instance)
   - [ttpe] select use of initial value of tts:position versus tts:origin
   (1 instance)
   - [ttx] select between tt element and region element for root of style
   inheritance (1 instance)
   - [ttx] select between version specific transformation helper class
   methods used during ISD transformation processing (24 instances)

In total, 85 (or more) code branches are determined by a document's
effective version.