stresses and k+1-order and k+2-order, k>=1, convergence for stresses and
displacements resp.

  L2-projection of the bulk displacements on the skeleton (TO-DO)

* Found issues related to some of the terms in the weak formulation.
  (TO-INVESTIGATE)

…p/GridapHybrid.jl into linear_elasticity_symmetric_stresses

…elasticity_symmetric_stresses

symmetric stresses HDG formulation for linear elasticity runs bottom top
without fatal errors.

Next step manofacture exact solution in FE space and check
consistency/correctness of the current implementation

* HDG formulation with symmetric stresses for linear elasticity,
  still without P_M projection, validated for solution in FE space
  and analtyic solution from paper.

* Next step: implement P_M projection.

Surprise, surprise. It crashes. To DEBUG.

* Added necessary fixes to support triangle meshes.