Remove redundant calculations of cell capture face lengths

[MattRolchigo]

The value triangle_dist calculated during a cell capture event is equivalent to the negative of the critical diagonal length for cell capture - the critical diagonal length can be used in its place, provided that the negative of the grain unit vector values are used to cancel out the sign change

For large problems/small resource sets, and particularly on CPU, this change results in up to a 2% performance improvement. For most problems, the improvement is negligible (1% or less)