WIP: Self Organizing Map

[thomasneirynck]

Example visualization to help validate #11786.

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This initial check-in is somewhat of a hamfisted way to shoe-horn in the Ponder application (https://github.com/thomasneirynck/ponder) as a visualization in Kibana.

But we can use it to illustrate the following use-case:

• can we store visualizations, independent of the ES-query, in the saved-object? Then, we can restore the visualization as-is in the Dashboard. The idea here is that the editor-phase is a design-phase that is data-driven, and the resulting visualization (ie. the map) is the product of this editor-step. In visualization-only mode (ie. the Dashboard), this map needs to be resurfaced and should function independently of any data-updates. In the case of SOM, the size of the resulting map can be quite large (order of MBs).
• optional: develop custom Editor (although for now, this PR is using the default side-bar editor)

[thomasneirynck]

closing this, as #11786 is ready