Generate mesh output from GML input

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The output can be specified in the output section in the prj file as follows:

    <output>
      <type>VTK</type> <!-- usual specification -->
      <prefix>1e0_{:meshname}</prefix>
      <suffix>_ts_{:timestep}_t_{:time}</suffix>
      <meshes>
        <mesh>cube_1x1x1_hex_1e0</mesh>
      </meshes>

      <geometrical_sets>
        <!-- example for point output -->
        <geometrical_set>
          <name>cube_1x1x1_geometry</name>
          <geometry>left_front_bottom</geometry>
        </geometrical_set>

        <!-- example for polyline output -->
        <geometrical_set>
          <name>cube_1x1x1_geometry</name>
          <geometry>vertical_polyline_left_front</geometry>
        </geometrical_set>

        <!-- example for surface output -->
        <geometrical_set>
          <name>cube_1x1x1_geometry</name>
          <geometry>left</geometry>
        </geometrical_set>
      </geometrical_sets>

      <variables>
        <variable> pressure </variable>
        <variable> v      </variable>
      </variables>
    </output>

An appropriate GML file has to specified in the prj file that contains the referenced geometrical objects. For instance:

<OpenGeoSysGLI xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ogs="http://www.opengeosys.org">
    <name>cube_1x1x1_geometry</name>
    <points>
        <point id="0" x="0" y="0" z="0" name="left_front_bottom"/>
        <point id="1" x="0" y="0" z="1" name="left_front_top"/>
        <point id="2" x="0" y="1" z="1" name="left_back_top"/>
        <point id="3" x="0" y="1" z="0" name="left_back_bottom"/>

        <point id="4" x="1" y="0" z="0" name="right_front_bottom"/>
        <point id="5" x="1" y="0" z="1" name="right_front_top"/>
        <point id="6" x="1" y="1" z="1" name="right_back_top"/>
        <point id="7" x="1" y="1" z="0" name="right_back_bottom"/>
    </points>

    <polylines>
        <polyline id="0" name="vertical_polyline_left_front">
            <pnt>0</pnt>
            <pnt>1</pnt>
        </polyline>
    </polylines>

    <surfaces>
        <surface id="0" name="left">
            <element p1="0" p2="1" p3="2"/>
            <element p1="0" p2="3" p3="2"/>
        </surface>
    </surfaces>
</OpenGeoSysGLI>

Updated xml snippet according to the reviewers recommendations