Canonical Composition Model

This release contains lots of changes to the core Photon library, including, a canonical object model for a composition, support for multiple namespaces for CPL, PKL and AssetMap, separation of logic to validate constraints on a Composition from the canonical object model, more tests for the IMPValidator, support for single track conformance etc. It is highly recommended that users move to this version at the earliest to obtain all of the above mentioned features.