Add initial (WIP) support for external schemas.
(1) rename TTML10 to TTML1;
(2) add external schema support;
(3) add smpte models (st2052-2010, st2052-2013);
Update documentation.
Enhance smpte verification.
Finalize support for SMPTE (ST2052) models.
General merge from master, to pick up SMPTE model support.