Fix radius/diameter confusion in overlap search.

Fix radius/diameter confusion in overlap search.

Drill sizes are specified as hole diameter, but the nearest neighbour
search is performed by distance.

The overlap relation is documented as "when the centre of one hole is
inside the other hole" which implies that we should be searching on
hole radius.  Presently the diameter is used, which is twice as far
as expected.

For holes that are the same size this is a minor technicality: such
holes that are apart by less than the hole diameter do "overlap" in
the drilled portion.  But when a small hole is near a large hole they
can be obviously disjoint yet slotifier can erroneously merge them.

Adjust the search to half the diameter, plus some slop (5 tenths)
so that holes exactly on the boundary are counted as overlapping.