catalog nulls

I wrote the introduction for my paper with Joey Richards (wise.io). It is all about how to use the null values you get in standard multi-epoch imaging surveys when a source is undetected at some epochs. We are working on the (common) case in which you get no detection and you also have no (easy) way to reconstruct the detection threshold or any upper limit information. If you ever thought such null values would be useless, and that they ought to be discarded, it turns out you are wrong (although the use of them is not trivial).