Documentation.
Added a get_tet function to get the neighbour of a tetrahedron.
Added a "crossing" label to edges and nodes (and deleted locked and stable labels).

A crossing means where two or more interfaces meet.
Updates the "crossing" label.
Does not move the interface if it is crossing.
Commented out relabeling.
Added a function for thickening the interface.
Minor change in deform function.
Bugfix in split of tetrahedron.
Added thickening of interface to resize_complex function.