Add support for Web Mercator WMS layers in Cesium.

Make `GeoDataCollection` look at the WMS server's supported coordinate reference systems in the `GetCapabilities` document (or `init_nm.json` for data sources added that way), and use any of the following, in order of preference:
* EPSG:4326 (standard Geographic)
* CRS:84 (alternate name for standard Geographic)
* EPSG:4283 (Australian system that is equivalent to 4326)
* EPSG:3857 (standard Web Mercator)
* EPSG:900913 (older name for Web Mercator)

If none of these is listed as supported, we request EPSG:4326 and hope for the best.

Previously Cesium always used 4326, so some layers would work in Leaflet (which uses 3857) but not in Cesium.  Now Cesium supports all the layers that work in Leaflet, plus some.

This pull request always switches to using Cesium's built-in support for WMS feature picking (see AnalyticalGraphicsInc/cesium#2057) instead of custom code in AusGlobe.