add magnetoelastic Kernel

Adding terms of the form K_ijkl m_i m_j m_k m_l (6-fold cubic anisotropy) and

B_ijkl m_i m_j e_kl (cubic magnetoelastic anisotropy)

tests on La2/3Sr1/3MnO3 [suitable film strain should give four-fold (in-plane) anisotropy]