recalculate feature envelope while exporting CityGML with LoD-Filter

[yaozhihang]

While running a CityGML-Export, the envelope information of each feature are currently read from the ENVELOPE column of the CITYOBJECT table and dann propagated to the Cityobject's boundedBy property. This may result in an inconsistency problem when the LoD-filter is activated.

For example, a CityGML-Building dataset could have LOD0 and LOD2 representations in the database at the same time. When exporting Buildings with only LOD0 representation, the envelope of each exported CityGML feature should not be retrived from the CITYOBJECT table directly, but shall be recalculated based on the LOD0 geometries of the respective building object.

[clausnagel]

Implemented with #239