Fix bugs with mixed-degree Lagrange/Bezier cells

This fix 3 things with mixed-degree Lagrange/Bezier cells (one for each commit)

• We have realized that Bezier Hexahedron of degrees 1-1-2 and 2-1-1 was working, but not with degrees 1-2-1 . See attached file to reproduce the bug box121_geom.vtu and the following render picture that is supposed to be a cube

The problem was that some cells are reused multiple times, and the parametric coordinates was only updated when its size was different than the number of points of the cell. But with multiple degree, for instance a cell of degree 1-2 will have the same number of points than a cell with degree 2-1. The fix consists in resting the parametric coordinates when different degrees are set.

• A second commit fix a potential bug that was writing a new parametric coordinate at the end of an existing one, instead of overwriting it.

• The third commit add the support of multi degrees to the node numbering converter from VTK8 to VTK9. This was initially not supported because multi degree was officially introduced with VTK9, but internally we have start to use multi degree before the the node numbering change was introduced, so we have some existing file that still cannot be converted.