Move config checking code from the ociocheck app into the core library

[doug-walker]

The ociocheck command-line app currently provides the most thorough validation of a config. Some of these checks would be quite useful for a variety of apps that use OCIO (and ocioview would be a prime example). But in order to make use of them, the checks should move from the ociocheck app itself into the core library where they are accessible from the API.

The current validate method on the Config class is very useful as a quick method of checking a config, but it does not try things such as resolving and loading FileTransforms (ociocheck loads all FileTransforms). It would be best to keep validate a quick check since that is already widely used by client apps. So any time-consuming tests or tests that require loading external files should be put in a separate function/method.