Remco work on scripts

- Changed the import_measure_xls script: if a row has six values instead of five,
  the sixth is assumed to contain the water level difference for the 1:250 protection
  level.

- Removed unused ReferenceValue model.

- Changed the import_vertex_xls script: headers may now start with the year (2050
  or 2100) followed by a :, and this year value will be saved with the VertexValue.
  If no year is present, assume 2100.