Custom scales system

[floledermann]

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D3's scale system has proven to be insufficient for our purposes. We need a scale system that fulfills the following needs:

• introspection of scale objects to find out about their properties (for legend generation etc.)
• strict handling of undefined and invalid values
• support for "nested" scales, e.g. having an ordinal scale taking care of undefined and invalid/special values, and a numeric scale for valid values
• support for multivariate scales
• support for setting up the scale (domain, range) from actual data through a callback (for things like Jenks natural breaks, quantile scales, ...)

Also see #12, #25

[floledermann]

TODO: investigate if D3 v4 offers improvements in this respect.