Geomaps API: add getGeoCoordinate() (#645).

New in Geomaps API:
{{{
GeoCoordinate getGeoCoordinate(Topic geoTopic)
}}}
Returns the geo coordinate of a geo-facetted topic (e.g. an Address).

New model class:
{{{
de.deepamehta.plugins.geomaps.model.GeoCoordinate
}}}
A long/lat value.

See #645.