gh_distance for approximate GH-to-GH distance [OLD #25]

[MichaelChirico]

Various options:

1. distance from centroid to centroid (d(A, B) = 0 iff A=B)
2. minimum distance (as close as possible within GH A to GH B; d(A, B) = 0 for all B neighboring A)
3. maximum distance (as far as possible within GH A to GH B; d(A, A) > 0)

Only the first is a "proper metric" for the reasons laid out in parentheses.