Metadetection is using meas_deblender just as a fast way to generate child records from multi-peak footprints consistently.

We should factor out the child-creating code (or at least interfaces and guarantees) from existing deblenders, and provide that functionality via either a trivial deblender task or some other kind of utility code.  Given that Scarlet and meas_deblender don't have a common interface anyway, trying to make this a task probably doesn't make sense.

This should include a way to make a catalog of just children, with no parents.