Splitting according to an objective function

[MattEdwards94]

Need to be able to split a grid according to an arbitrary objective function.

How does this work for multi-level grids?

Perhaps:

1. Loop over the coarse grid of cells.
   i. Check if cell has children
   yes:
   initialise loop over 4 children, return to (i)
   no:
   Decide whether to split cell or not.

This would work, though I think could be slow perhaps considering it would be just a loop with no vectorisation. Can't see a better way for now