Allow Extent to be multipolygon

[FHunt-OS]

It would be useful if Extent could be a multipolygon to allow query results from non-contiguous areas, without needing to make multiple extent/API objects.

[E-Paine]

This is not easily solved, at the moment. The underlying API does not support MultiPolygons or multiple geospatial filters. Therefore, a significant amount of post-processing would need to be done, which breaks away from the idea of this being a fairly thin wrapper of the API. This issue should stay open for when this is implemented upstream.

[ian-astun]

Since it only takes the extent of the polygon it should be easy to make it take a multipolygon. I can filter out the bits I don't want later if necessary.

[Ilkka-LBL]

A related issue is that using Extent.from_ons_code() method doesn't work because some ONS codes are interpreted as multipolygons. One example is that of Greenwich (E09000011), which shouldn't even result in a multipolygon.