- adapt multiple trace attributes.js files accordingly
- standard colorscale/attributes import to colorScaleAttrs

- 'line' is now declared within _module.colorbar, so
  we don't need this block
- add scatter3d test for good measure

- reuse Colorscale.supplyDefaults
- use 'cmin', 'cmax' and 'cauto' no matter the trace types plotted
- keep track of color axes referenced in the traces in
  fullLayout._colorAxes
- ignore coloraxis referenced when colorbar visuals are incompatible
- tweak PlotSchema.get() for layout.colorscale / layout.coloraxis?
  edge cases

... to help with per-trace min/max/auto or per-coloraxis defs

... and add makeColorScaleFuncFromTrace wrapper to DRY things up
    a little bit.

- use min of min (and max of max) of all traces linked to same
  color axis to compute min/max when auto:true
- call Colorscale.crossTraceDefaults after having relinked layout,
  to use coloraxis._min, coloraxis._max
-

- which has special logic which currently mutates zmin/zmax after
  the calc step.

- remove zmin/zmax mutations happening after the calc stpe
- make traceIs('contour') traces call Colorscale.calc with
  their own options.