Add support for custom color maps, some predefined ones

[Vindaar]

I recently wanted to compare Viridis with Plasma for a plot and change the colormap to be white as the zero value. Thought I'd clean this up and add support for easier handling of custom colormaps.

Instead of playing around with the JsonNode after converting to PlotJson (or serializing), one can either assign one of the predefined custom colormaps:

• Plasma
• Magma
• Inferno

each with and without zero value as white

• Viridis with zero as white
• white to black instead of plotly's "Greys", which is black-to-white) by using the

  trace.customCmap = getCustomMap(<field of PredefinedCustomMaps>)

  or create a CustomColorMap object and assign it. The fields of it are rawColors, which are just a sequence of tuple[r, g, b: float] in the range of 0.0 .. 1.0. The custom maps can consist of N values. They will be taken to cover the whole data range. The colormap range can additionally be restricted (or enlarged) via the zmin and zmax fields (only for heatmap, was not sure if I should add it for Countour?).

The normal colormap field has to be set to Custom for the custom map to be considered.

Alternatively the colormap sugar function can be used on a Plot[T] object. This is more convenient imo. One can hand one of:

• ColorMap: enum: Plotly's default colormaps
• PredefinedCustomMap: enum on of the predefined custom maps
• CustomColorMap a custom colormap object
• rawColors a sequence of raw colors as shown above

See the fig20 example and the test case for explicit usage.

[Vindaar]

Renamed the fields.