Modern Camera spec #584

IECore::Camera is fairly limited in its features, and we often have trouble fully representing DCC camera settings. We often resort to baking things down, and hence lose information. Lets update to a more modern spec that can handle the common DCC settings without baking it down to screenWindow when possible.

Related to #29 in that Alembic has a camera spec we may consider adopting or at least need to support loading.