0 cohsnr on some triggers in 3 detector runs.
We've seen some runs with 0 cohsnr despite having non-zero snglsnr on at least one detector.
The cohsnr calculations in postcoh_kernel.cu
look sane, so it's likely that the issue is the 'umap' that's used for cohsnr with 3 detectors. We have a run with debug that should show the issue. (same as #123)