`modelToHeavy` was modified to remove an incorrect thresholding
algorithm used to truncate convolved scarlet models and replace
it with an algorithm that grows the deconvolved footprint with
the rectangular PSF.

Deblended children near the edge of an image can have their
footprints dilated outside of the exposure by the PSF width.
This commit ensures that the dilated model is always contained
fully in the exposure. This is valid, as later in pipe_tasks
the science pipelines flag edge sources so it isn't expected
that all of their flux is contained in the image.