Beta release of v3.0.0-beta.16

Breaking changes

• Move scale rangeStep property to a nested range.step property ("range": {"step": 20}).

Other changes:

• Add range scheme count to discretize color schemes for quantile and quantize scales ("range":{"scheme": "plasma", "count": 7}).
• Add support to include raw domain value arrays (or array-valued signals) in multi-domain scale definitions (For example: "domain": {"fields": [{"data": "table", "field": "foo"}, ["a", "b", "c"], {"signal": "values"}]})
• Update legend label formatting to show value ranges for quantile, quantize and threshold scales.
• Update example specs to track changes.