Infinite loop in feature detection with PBCs when all points below/above threshold

[freemansw1]

This is definitely an edge case, but it got me. In RC_v1.5.0, if all points are below/above the threshold (if target is minimum/maximum), an infinite loop (or at least a very long loop) can occur when trying to process periodic boundary conditions.

I've marked this low priority for now as I don't think it should hold up v1.5.0 release as it's pretty unusual, but wanted to note it.

[w-k-jones]

Does this only happen with PBCs or with any case? We should probably check and raise an exception if all points are above all thresholds in any case