implements a cast from polyMesh to Mesh and vice versa using static_cast(polymeshInstance) or static_cast(trimeshInstance) as mentioned in #23 (closed) .

Copy constructors for respective mesh types were added as well as a function to copy all properties from one mesh to another. Unittest for mesh conversions were added checking the number of vertices / faces as well as custom double vertex properties and edge properties.

Note: The used triangulation only works for convex polygons, it should be replaced by the earclipping triangulation mentioned in #33