Map splitting

[Danielv123]

Should be fairly trivial in comparison to merging.

1. Upload a normal map in the web ui
2. Extract map exchange string and create a gridworld based on that
3. Prepopulate the gridworld using the uploaded map as a base. Delete all chunks outside of the normal grid square area.

A major challenge I forsee is figuring out how much of the map to pregenerate. I suppose just picking all squares with inserters, belts or rails should do pretty well. Another option is simply to ask for a number squared.