Work on showing units, fixing a bug with hidden layers and the cache script.

- Add a 'visible_criteria' method to datasources, that by default just
  returns chooseable_criteria(). This should be used to hide choices
  that shouldn't be shown to the user. The previous method of hiding
  them from visible_criteria was inadequate, because then they would
  be hidden from internal use too (e.g., the caching script couldn't
  find them either).

- Add a 'unit' method to datasources, that optionally returns a string
  describing the unit of the data represented by
  them. Grondwaterinbrabant-site issue #10.