warn if we try to set geometrical interpolation matrices in model-based views:

we should actually not allow it, as it makes no sense. 

we should reserve the additional matrices to implement other (e.g. vector) 
interpolation schemes

Post/PViewDataGModel.cpp

@@ -152,6 +152,9 @@ bool PViewDataGModel::finalize(bool computeMinMax, const std::string &interpolat
         }
         else if(it->second.size() == 4){
           // use provided matrices for field and geometry
+          Msg::Warning("You should not specify the geometrical interpolation "
+                       "in ElementNodeData: the geometry is completely determined "
+                       "by the mesh element type. This feature will be removed");
           setInterpolationMatrices(it->first, *it->second[0], *it->second[1],
                                    *it->second[2], *it->second[3]);
         }

doc/texinfo/gmsh.texi

@@ -3779,10 +3779,10 @@ for prisms, 8 for hexahedra, 9 for polygons and 10 for polyhedra.
 
 @item @var{number-of-interpolation-matrices} 
 is the number of interpolation matrices provided for this element
-topology. Currently you should provide either 2 or 4 matrices: the
-matrices have the same meaning as in @ref{Post-processing commands}.
-The matrices are specified by 2 integers (@var{num-rows} and
-@var{num-columns}) followed by the values.
+topology. Currently you should provide 2 matrices, i.e., the matrices
+that specify how to interpolate the data (they have the same meaning as
+in @ref{Post-processing commands}).  The matrices are specified by 2
+integers (@var{num-rows} and @var{num-columns}) followed by the values.
 
 @end table