better supports re-entrant use, especially for the use case where
the import was abandoned before saving, or the generated cells
were not saved (which is easy to do, because the top level cell
is always written out to disk, but the rest of the cells will only
be written when "writeall" is executed). The new code detects
input cells that did not have any layout file (has flag "not-found"
after loading) and deletes them and their instances so that they
can be regenerated from scratch.
Tim Edwards
eda5f506f3