Further work on refactoring the camera API. Added proof-of-concept implementation into MprRenderer and MprDemo. Thereby fixed various issues of the new CameraData API that occured:
* tgt::Navigation::updateClippingPlanes() yielding NaNs if the scene bounds were not defined
* NumericProperty yielding endless loops in case of NaN values
* Updated AbstractProcessor::process() to automatically validate level XYZ after calling updateXYZ().
* Adapted changes through renaming tgt into cgt

refs #141
refs #386