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?

该 issue 涉及 OpenColorIO 中的 YAML 序列化。首先定位 YAML 序列化的入口点，并确定默认值的表示方式；当存在一种明确的机制，可以省略保持默认值的值，同时保留显式指定的非默认值时，即可认为完成。

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