Fix and improve demo data

[olivierdalang]

• [ ] fix (interlis-)invalid data (see https://github.com/QGEP/qgepqwat2ili/blob/master/qgepqwat2ili/data/test_data/qgep_demodata_hotfix.sql)
• [ ] add missing data (e.g. KEK related tables have no data, and thus are not tested)
• [ ] include or exclude extensions depending on whether they are considered part of teksi_wastewater or not (SWMM, audit table, roles)