Dear Yves
Why you are describing it as
/* ******************************************************************** */
/* Element RT0 on parallelepideds. */
/* ******************************************************************** */
I became concerned over a word parallelepideds.
Does it mean that element works only for affine transitions? Like in 2D case the reference unit square element is transferred to a parallelogram but not to a trapezoid. Does it mean that passing of the trapezoidal test described in Section 6 of MIXED FINITE ELEMENTS FOR ELASTICITY ON QUADRILATERAL MESHES paper by DOUGLAS N. ARNOLD is not expected?
You(author) don't use the Piola transformation that should work fine so i'm wondering is there a reason for that?
I have made a hardcoded implementation of more precise procedure(based on Piola transformation) described in the paper mentioned above so now trapezoidal test passed. Let me know if you are interested in incorporation of it.
Best Regards, Egor