Change bootstrapping -> jacknife / LOOCV?

[ericward-noaa]

Unclear why the original code / approach for this was doing bootstrapping, but leaving observations out in turn seems like it might be more along the lines of what we're after. Should ask Jameal about orig motivations...

See here:
https://github.com/raine-detmer/Threshold-Detection/blob/5f6312e18c4b0c004b09ea739adb2cd1a2bdcf4e/functions/bootfun.R#L53