Adding much faster, more scalable iterative distance field function.

Basically, by making each pass a true sweep in one direction, this version
(each axis each direction, e.g. up/right/down/left in 2D) propagates
reasonable values to all mesh nodes in each iteration, and accuracy
improves quickly.  The old algorithm required ni+nj+nk iterations for
much accuracy at distant mesh nodes, this one gets almost as good
accuracy in 3 iterations for any mesh node.  This algorithm still does
not have high accuracy for points far outside the target mesh extents.

However, you can now choose the much slower full-accuracy O(m*n)
algorithm.