When creating the search structures in PointLocatorUniformGrid, a point
outside the boundary would be given an invalid bin id. These points
could never be found. Generally, this is not a big deal for points
outside of the boundary, but it could be a problem for points on the
boundary. A point on the boundary could be taken as outside the
boundary. Since the boundary is chosen from limits of the points, some
will almost always be on the boundary.
Fix this problem by clamping all points to the nearest valid bin. This
could cause a problem if the user has selected a boundary excluding a
lot of points. All those points could be grouped to the same edge bins,
but that is probably not a great idea anyway.
