Add pp_packing to list of pep8 exclusions.

Since it's a generated file, there's not much chance of us fixing it.
This file is not always created though, so the exclusion needs to be
made optional.
Change config creation so that udunits loads.
Write ungzipped files to a temporary path if needed.

This may occur if the original test data are installed in the system,
and consequently, the ungzipped version cannot be placed in the same
directory.