improve lattice refinements

known bugs / things that need to be done

• throw an error if the refinement is applied to a non-triangular tiling
• after the refinement, polygon indices are set to zero
• think about neighbour search; simple radius search fails
• update neighbour graph on the fly during the construction of the refinement would be the most elegant way
• use geodesic distances for the cuts, rather than Euklidean

@fdusel