ENH: Enable Droptip on tree (without data table).
ENH: Add ensemble data model item for treeheatmap vis.

When running tree drop tip analysis on a tree with data table,
the resulting pruned tree and table pair is listed under the
ensemble data model item with the tree heatmap view.
Merge branch 'DropTip'

* DropTip:
  ENH: Add ensemble data model item for treeheatmap vis.
  ENH: Enable Droptip on tree (without data table).