Include new `coverage` field in `NwBibSpatialQuery`

We display `coverage` as `Raumsystematik`, so we should make it
searchable too (`nwbibspatial` query param, `Raumsystematik` in UI)

See https://github.com/hbz/nwbib/issues/66
Merge pull request #67 from hbz/nwbib-66-coverage

Include new `coverage` field in `NwBibSpatialQuery`