au: split unit and housenumber

[missinglink]

I noticed some of the OA sources for AU come with the unit/housenumber concatenated:

While its difficult to accurately split these slash delimited values internationally, it should be simple enough to do on a per-country basis.

There's already a conditional statement for AU which can be used to detect sources appropriately.

ref: https://auspost.com.au/content/dam/auspost_corp/media/documents/Appendix-01.pdf

[missinglink]

actually this may be better implemented in pelias/model as a post-processing script, which would allow it to work for any importer which produces records on the address layer, we could use the country result from PIP to decide on which strategy to use.

[edit] on reflection, it's mostly an issue with OA since OSM mappers usually split these fields out, we can port it over later if/when required.