Adding ability to CodeGens to post-process CodegenModels

[GottaGetSwifty]

Description of the PR

Added a post-processing method for Models.

This allows a CodeGenerator to add/process a Model's properties without overriding core functionality/methods.

While this could be done by subclasses overriding fromModel, the postProcessing model is used elsewhere and improves clarity as to how that type of change should be implemented.