gf_eltm — get a descriptor
ELTM = gf_eltm(elt_matrix_type, args..)
gf_eltm('base', Fem FEM) return a descriptor for the integration
        of shape functions on elements, using the Fem FEM.
        
gf_eltm('grad', Fem FEM) return a descriptor for the integration
        of the gradient of shape functions on elements, using the Fem
        FEM. 
gf_eltm('hessian', Fem FEM) return a descriptor for the
        integration of the hessian of shape functions on elements, using the
        Fem FEM. 
gf_eltm('normal') return a descriptor for the unit normal of convex faces.
gf_eltm('grad_geotrans') return a descriptor to the gradient matrix of the geometric transformation.
gf_eltm('grad_geotrans_inv') return a descriptor to the inverse of the gradient matrix of the geometric transformation (this is rarely used).
gf_eltm('product', Eltm A, Eltm B) return a descriptor for the
        integration of the tensorial product of elementary matrices
        A and B. In order to obtain a
        numerical value of theses matrices, see
        gf_mesh_im_get(MIM,('eltm').