Running `matchedVisitMetrics`for DM-17830 was yielding AM1 of 0 for HSC WIDE tracts 9615 and 9697 which have visits numbers in the single digits. AM1 is the median of the RMS of the distances between 2 stars in N visits. Closer inspection revealed that more than half of the RMSs were exactly zero, which is why the median was exactly zero.  Michael Wood-Vasey quickly found the problem which was that if a pair appeared in exactly one visit, the stdev([one distance]) was exactly 0  and not NaN.

Require at least 2 distances before computing their stdev.

Michael Wood-Vasey will investigate the potential bias is in the median RMS as a function of N.

This change will increase our reported AMx reported in squash.