Provide YAML default mechanism to write fewer values

Right now, yaml serialization often spits out all values regardless of whether or not they are 'set'. While this is explicit and easy to understand, it makes for lengthy files. Perhaps in some cases it is more appropriate to omit the value unless a non-default is specified?

The issue concerns YAML serialization in OpenColorIO. Start by locating the YAML serialization entry points and determining how default values are represented; done should mean there is a defined mechanism for omitting values that remain at their defaults while preserving explicit non-default values.

Written by the indexing model from the issue text.