CalcHessian for 2D triangle is not implemented when mesh is linear.

[sshiraiwa]

This is memo based on conversation with @mlstowell.
I realized that when using 2D unstructured triangle mesh, calling CalcHessian fails because the method
is not implemented

MFEM abort: method is not overloaded
 ... in function: virtual void mfem::FiniteElement::CalcHessian(const mfem::IntegrationPoint&, mfem::DenseMatrix&) const
 ... in file: /home/sshiraiw/twopi_roots/20220725/src/mfem/fem/fe/fe_base.cpp:104

It is because some class in fe_fixed_order does not have CalcHessian implemented. In fact, changing the mesh to
curved mesh resolve this issue (but it computes Jacobian of element transformation).