Keep spatial coverage if not a bbox to enrich geometa with bounding polygons

[eblondel]

The spatial coverage may be a simple bbox wkt but it could be also a more complex spatial coverage (eg. simplified polygons, simplified lines) less reductor than a bounding box. At now geoflow reduces the WKT into a bounding box, which results in a loss of meta information. Some logic should be added to keep such more detailed spatial coverage. In geometa action it will result in setting bounding polygons, in addition to the geographic bounding box.