Fix spec issues

[vanous]

This is a big rework of the library. There are some big breaking changes, consumers will need to adopt, it should be fairly simple.

• implemented all MVR Spec 1.6 defined objects
• implemented to_xml or populate_xml everywhere
• reworked structure to match the XML definition:
  • container classes are now python classes
  • several previously not existing objects now exist (scene)
• updated all tests