Add copy ctor to ofxAssimpModelLoader, fixes #2826

Takes the paranoid approach of just reloading the original loader's file. This is to address the problem of not being able to make a simple `vector<ofxAssimpModelLoader>`. It's not very efficient, since it reloads the original model loader's file instead of doing something like copying the in-memory resources. This is because the `aiScene` deletes all its resources on destruction without doing something like reference counting to see if anything else wants to use said resources.

The warning log should only fire in the case where the original model loader has a scene, but doesn't have a file (either the file moved, or it was initialized with an `ofBuffer` in the first place).