The following changes to functionality are in this commit:

    - 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.