materi_maxwell_stress number_of_chains

Maxwell stress ${\sigma_{11}}^m \; {\sigma_{12}}^m \; {\sigma_{13}}^m \; {\sigma_{22}}^m \; {\sigma_{23}}^m \; {\sigma_{33}}^m$ is added to the node_dof records. The parameter number_of_chains should match data item group_materi_maxwell_chains. The number of maxwell stresses is 6 * number_of_chains. Condition: the -materi differential group_type should be present.