Possibility of deleting layers with empty citydb schemas

[calsgeo]

It should be a possibility to delete existing layers that were created before dropping a the 3dcitydb schema. If the user decided to not drop the existing layers before cleaning the schema it work be allowed to do so. For example:

1. User imports the data to an specific schema inside the DB
2. User creates and refreshes the layers of the corresponding schema
3. User clean the schema, like using the cleanup_schema() function
4. when trying to connect from the plug-in to the schema it won't work since it is empty but the mat views are still created and populated.