More gap checking, and culling

Now in this version the gap checking has been refined to only test against neighbouring cells that share edges - ie left, right and if pointing up then down or if pointing down then up.

This first set begins with 1/4 randomly on and an off count less than 2 - ie a gap is filled if at least 2/3 of it's neighbours with shared edges are on.

Here are some pairs showing starting configurations and once gaps have been filled. I have now switched to checking and counting if on, rather than off, as this seems slightly simpler and more natural to describe. If a neighbouring cell is out of frame it is still off. Also to speed things up, I am now running gap check only for cells that are off.

In the next set of pairs I have added culling - after each gap checking loop, every cell that is on is checked for isolation and removed. Isolation is determined here by having no neighbour cells that share edges on.