Add processing of `census` fixtures

[griff-rees]

The lwmdb census.model is the last data element as yet unimplemented in alto2txt2fixture.

• Look up the files originally purged from lwmdb as a starting point
• Assess the viability/time estimate
• Any necessary alterations to the census.model
• Decide whether to target csv/json or geojson/shapefiles
• Tests for minimum example
• Generate files
• Test importing those into lwmdb
• Document

[griff-rees]

See census branch