Initial development on a periodic slab.
Removed distance calculator dependence on structure which was unnecessary.
Bunch of improvements to voronoi slab generator.  More to come.
Just about got periodic voronoi slab to work.  Changed a couple of things in tpsd geom optimiser.
Working version of periodic voronoi slab.
Fixed a bug in the way atoms were laid down in the voronoi slab generator.  Now the order is as we expect.