fix bug 2D elements having different vertex than 3D element due to multiple vertices on degenerate edge

When going to high-order elements, a 2D element matching a face of a 3D element could have one different vertex due to creating vertices on degenerate edges.

See this example:

SetFactory("OpenCASCADE");

A = 1; B = 0.5; Np = 50;

For i In {1:Np} angle = Pi/2/(Np-1); Point(i) = {A*Cos((i-1)angle), BSin((i-1)*angle), 0.}; EndFor Point(101) = {0, 0, 0};

BSpline(1) = {1 ... Np}; Line(2) = {1, 101}; Line(3) = {101, Np};

Line Loop(1) = {1, -3, -2}; Plane Surface(2) = {1};

Extrude { {1., 0., 0.}, {0., 0., .0}, -Pi/2 } { Surface{2}; Layers{3}; Recombine; }