Mesh2 shape export

Introduces functionality to convert a 2D mesh to a SHPT_POLYGON shapefile. This is functionality requested by AS and others for groundwater simulation purposes (coupling to JAMS, etc.)

- only 2D meshes can be converted
- only triangles and quads will be considered, lines are ignored
- each element is written as a polygon (SHPT_POLYGON) into the shapefile
- reading results has been tested with ArcGIS and OGS and should also work with any other programme supporting shapefiles
- a DBF file is written containing information from the original mesh: element ID, material ID, the first three nodes of the element (for identification later on; three nodes are all nodes of a TRI and enough nodes to ID a QUAD)