Remove refine_cell methods from TriangleCell, TetrahedronCell etc.

[Chris Richardson]

There are lots of possible refinement algorithms, and it doesn't make sense to have refinement as a method of a cell type. Any useful refinement code can be migrated to the refinement folder.

[Prof Garth Wells]

Could you deprecate the functions for 1.6 so we can remove them in 1.7? Or are they sufficiently low-level to just remove?

[Chris Richardson]

I'm not sure anyone is actually using them directly(?) - they are called by the uniform refinement methods in serial. It would be good to deprecate them, if they are called directly - I'll see what I can do that makes sense...

[Anders Logg (Chalmers)]

Removing milestone: 1.7 (automated comment)

[Jan Blechta]

33148ae8309a6f59c88f9cc85b61837f0858bc7b