streamthinca: include background in "non-coincident singles"

- on each invocation of the iterative coincidence code, a list of the singles that have not formed coincidences is returned.  this patch changes the interpretation from "singles that have not formed coincidences" to "singles that have not formed zero-lag coincidences".  i.e., singles that formed time-slide coincidences are now included so that they'll be included in the denominator histogram of (snr, \chi^2) values