- Removed hardcoding of options.

- The cache script now creates all layer objects, but only downloads
  values for layers of which the latest value is used by something
  (coloring).

- Coloring is done using the configuration in the ColorFromLatestValue
  model.

- Percentile bars are done using the configuration in the
  PercentileLayer model.

- Layers that are used for augmenting other layers (color layers,
  percentile layers) can now be hidden.