Fix building of queries requiring SELECT DISTINCT

[clausnagel]

This PR fixes #262.

In some cases, the query specified for a CityGML export leads to a SELECT statement that chooses the ENVELOPE column from CITYOBJECT and at the same time requires DISTINCT to avoid duplicates. While DISTINCT can be used for the ENVELOPE column under PostgreSQL/PostGIS, this combination leads to an error under Oracle (see #262).

This PR fixes the issue by avoiding DISTINCT in case the ENVELOPE column is selected. Instead, the query is rewritten to use row_number() and ORDER BY to avoid duplicates.