Read the mesh

[lsawade]

• Add material_id == 4 (Cosserat) branch in core/specfem/io/mesh/impl/fortran/dim3/read_materials.cpp. Read 8 Cosserat parameters from the Fortran binary stream and create a material<dim3, elastic_spin, isotropic_cosserat, none> object. Push a material_specification with (medium_tag::elastic_spin, property_tag::isotropic_cosserat, attenuation_tag::none)
• Update core/specfem/io/mesh/impl/fortran/dim3/read_materials.hpp if the function signature needs to change
• Update materials container in core/specfem/mesh/dim3/materials/materials.hpp — extend the FOR_EACH_IN_PRODUCT macro from MEDIUM_TAG(ACOUSTIC, ELASTIC) to include ELASTIC_SPIN