WIP filtering of WMS layers based on MaxScaleDenominator.
Filter data sources returned by CKAN.

The data sources that are shown must exist in the GetCapabilities response, and must have a MaxScaleDenominator larger than 1e10.
Also filter regular WMS GetCapabilities layers.
Merge pull request #379 from NICTA/vic-filterMaxScale

Filter out CKAN data sources with a MaxScaleDenominator less than 1e10.