Group symbolizer reference

[hollinger]

This adds reference information for group symbolizer (mapnik/mapnik#2160). The goal is to be able to create group symbolizers in Carto CSS. This effort also requires a corresponding update to carto (mapbox/carto#349).

See https://github.com/mapnik/mapnik/wiki/GroupSymbolizer for more information on group symbolizer.

[springmeyer]

planning on reviewing and testing this soon. Let me know if it needs any updates or if its still ready to go from your perspective.

[hollinger]

I am not sure that the above two attributes belong here in mapnik reference, because they do not correspond directly with options used by mapnik. The 'group-name' and 'group-class' css options are really aimed at the carto compiler, which will use the name and class just like a layer to build rules and symbolizers that go inside of the group symbolizer. I included them here mostly for simpler validation in carto.

[hollinger]

The fact that a group symbolizer can contain a whole set of rules and symbolizers is somewhat tricky to represent in the reference json and in carto syntax. It is possible that there could be a better solution than what I have here.

The concept of using child elements to define layout, format, and placement nodes was introduced in #62. If we go with that approach, it could possibly be extended to define the contents of group symbolizer. It might be better to get the work in #62 finalized before merging this.

[pnorman]

Any additional changes to this PR are probably going to have to be done by the Mapnik team, not @hollinger