update_obj

'update_obj': update the object graph representation and its projection field, record it in the uvmat interface
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Object=update_obj(UvData,IndexObj,ObjectData,PlotHandles);

OUTPUT:
UvData: data to be stored as 'Userdata' on the uvmat interface
IndexObj: object index for a new object added to the list in UvData
   the function updates UvData.Object{IndexObj}, and possibly adds a new plot (UvData.Plane or Line) in the list atached to the interface

INPUT:
UvIn: structure stored as 'Userdata' on the uvmat interface
IndexObjIn: object index for an existing objects stored in UvData
ObjectData: structure containing the input object properties
PlotHandles: structure containing the handles of the plotting parameter buttons on the uvmat interface (obtained by get_plot_handles.m)
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