Make Model::calculateModelProperties() functionality public

[boksajak]

Making Model::calculateModelProperties() public will enable Falcor users to write their own model loaders without need to modify Falcor.

This function needs to be called after creating a model to calculate many member variables (e.g. the radius). As it is currently private, one can only write a custom model loader by rewriting the Falcor code, which makes the code non portable without distributing the updated falcor itself.