undeprecate colorCalculator and fix the fishy code

Although scales and accessors are bright and shiny, they don’t cover all use cases.

In particular, it’s much easier to special-case a specific value using the colorCalculator, as seen in the venture capital example. I don’t know how to create a scale that returns normal values for everything except for one exact value (undefined in this case).

I had two reasons for deprecating this method:

1. It’s assigning to getColor, which is a horrible horrible thing for a setter to do
2. I thought that it’s always better to use a D3 object than to write code

The second reason isn’t all that good - D3 is not supposed to be a framework that stops you from writing code! Sometimes writing a little logic is better than the best-abstracted toolkit.

The first reason can be fixed without hurting efficiency too much.

[gordonwoodhull]

Released in 3.0.11; backported to 2.2.2 for all those folks still using d3v3!

[gordonwoodhull]

More recent versions of d3-scale include .unknown() and diverging scales, which cover some of the use cases for colorCalculator. However, a function will inevitably be easier in some cases.

I’m fixing this by adding an override function _colorCalculator, and removing the horrible assignment to .getColor, as well as the deprecation warning.