added check for holes in mesh (works only for square meshes)

[morishuz]

• wrote method that checks that mesh is square
• wrote method that can check for holes in a mesh if this is square (i.e. there is a vertex for each of the corners of the 2d bounding box). this should hold in most cases as meshes are generated from square rasters.
• added call to Mesh::check_for_holes_in_square_mesh() from Mesh::check_tin_properties()
• wrote test for Mesh::check_for_holes_in_square_mesh()
• wrote test for Mesh::is_square()