validation routines set the detection threshold to negative infinity in an
attempt to elicit all the possible detections for the purpose of computing mean
average precision scores.  This works fine when using the scan_image_boxes
object but causes the detection quality to degrade when the scan_image_pyramid
is used since this object performs random subsampling to avoid producing an
excessive number of detections.  The fix is to allow the user to set the
adjust_threshold parameter themselves to whatever value they feel is reasonable
rather than hard coding it to negative infinity.