Add support for Treemap charts

[FabienLelaquais]

Description
Support plotly.js treemap type (needs a new parents axis)

See Treemaps in JavaScript.

Acceptance Criteria

• Ensure new code is unit tested, and check code coverage is at least 90%
• Add relevant demos (doc/examples/charts)

The documentation part is taken care of in this issue.

[trantor]

+1