Add the ability to configure different WMS layers in <image src> URLs based on zoom level

[prushforth]

A WMS often has multiple layers of similar type, which are appropriate for display within different scale ranges (e.g. single- vs double-line streets), and the developer-user of the WMS has to become educated as to what layers to request depending on the scale of the map.

Given that a MapML request has a defined scale, it should be possible to know what layers a given WMS request should include. To do that, the MapMLServer admin (the 'developer-user' from the WMS client scenario above) would configure a set of URL templates, each member of which would apply to a zoom range of one or more zoom levels. The element would then receive a map image that is appropriate to the scale.

Note that scale-appropriate layering can be done on the server, and so would be unnecessary on the MapMLServer, but such a feature would be nice to have. It could even be used to 'federate' services into a single MapML service under some circumstances, for example a mix of tiled or image service request could be returned, depending on the zoom of the MapML request.