Output submesh specified by material ids

Elements of the corresponding material IDs of the mesh will be extracted to a separate sub-mesh. The bulk-mesh properties are transferred to the sub-mesh. The file name of the sub-mesh in the example below will be `bulkmesh_0_2_4

<output>
    ...
    <meshes>
        <mesh>bulkmesh</mesh> <!-- output entire bulk mesh -->
        <mesh material_ids="0,2,4">bulkmesh</mesh>  <!-- output only those parts with specified mat IDs -->
    </meshes>
</output>

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