As part of metadetect (and other uses!) we will be doing coadds in small edge-free coadd "cells". This ticket adds cells to patches in skymaps as a configuration option.
When this configuration is chosen, the patches are built up in units of cells, and the tracts are built as units of patches, so all patches and tracts contain an integer number of cells (default is 20 cells per patch in the inner region). All patches have an extra ring of cells around the inner region which sets the overlap boundary. This is very helpful to ensure that neighboring patches will have a subset of pixels that are exactly identical (unlike the current "legacy" tract building method).