Failed lookup does not fail, just do not generate Coords.

A failed lookup used to fail with an error message. That in turn,
disturbs treecutter. Now if Coordinates are not found a Coord element
is not generated. The None values in the Address are converted to ''
or filled by a successful geocode.
Functions c and s needs refining. Empty elements are kept or
destroyed with the clean_db function.

clean_db function can clean code to become invalid xml so
use carefully. clean_db and the s/c functions will have
to be put in a docbook/lxml module at some point.