The mapping for a `geo` context accepts four settings:

[horizontal]
`precision`:: This defines the precision of the geohash and can be specified as `5m`, `10km`, or as a raw geohash precision: `1`..`12`.

`neighbours`:: Geohashes are rectangles, so a geolocation, which in reality is only 1 metre away from the specified point, may fall into the neighbouring rectangle.  Set `neighbours` to `true` to include the neighbouring geohashes in the context.  (What's the default?)

`path`:: Optionally specify a field to use to look up the geopoint.

`default`:: The geopoint to use if no geopoint has been specified.