Full implementation ColorCategoriesStyle

[VictorVelarde]

I made several 'small' comments, but overall, the main idea LGTM. (explicit source instantiation with metadata + source-dependant style functions).

Main points for discussion, with things I miss:

• if we're gonna support stats for more than 1 field at the same time (so to allow several combined helpers, eg. color dependant on one field and size for other field). I would vote yes.
• If we extend the mechanism for the new 'style functions', that makes use of the Source, so they can allow a bigger object (Layer).

With probably a bit less of urgency / relevance now:

• to give the source methods a bit more of cohesion (getFieldStats, getGeometryType, etc.) with the use inside of an intermediate Metadata class. The idea is, instead of having smaller 'pieces of state' in the source class, we have a big chunk (this._metadata), updated after instantiation, and the rest of them are functions, extracting pieces from it (eg urls) or calculating (eg from stats)

[alasarr]

I agree with the three comments @VictorVelarde. Let's do it, they're not hard right now and they will be hard in the future.

[alasarr]

What I don't agree is to adapt to CamelCase the fields of the API. I like the idea to keep them as they are at the API and it requires no extra effort 😄

[alasarr]

Completed the implementation of the 2/3 points.

If we extend the mechanism for the new 'style functions', that makes use of the Source, so they can allow a bigger object (Layer).

I've not done this, because if we do that we create a circular dependency between Layer and Styles. Maybe we could add a map parameter with the instance of the map instead of the layer. Thoughts?