Optimize NURBS from deflectometry learning

Currently it is only possible to learn one NURBS surface from deflectometry data at once. Learning NURBS from deflectometry data was implemented before we parallelized the NURBS and other parts of ARTIST. We now have the possibility to learn multiple surfaces at once.
This has low priority, as the focus of ARTIST lies elsewhere and deflectometry is only used as a comparison but it could be done.