Added method for tidying loose ends.

* tidy_loose_ends() has been added to ChunkMap so all the loose ends can easily be removed.  I was thinking of adding weights to this, or putting it 'higher up' in the generator framework - but I think this can be used for more of a utility function for generators to use rather than something that's forced on them - though maybe the user could choose to run this as a post-processing pass.

* Updated the basic generator to call tidy_loose_ends()
Refactored neighbour_vectors into a static variable.