Adds two "full stokes" forms for P2 elements
Commit adds * p2_full_stokescc_affine_q3.[ch]pp * p2_full_stokesvar_blending_q3.[ch]pp While the first one ist more for internal comparison purposes (for constant coefficients the epsilon part of the full stokes would reduce to the vector laplacian) the latter one is the one we need for the anelastic mantle convection models. Commit extends the HytegVsFenicsFormTest for the two new forms.