Leaf icon for Tree View

[juliostanley]

Hi,

Based on the comment at
https://github.com/openfin/fin-hypergrid-data-source-base/blob/master/index.js#L17

The code at:
https://github.com/openfin/hyper-analytics/blob/master/js/DataSourceTreeview.js#L237

Should take into consideration, that __EXPANDED will be undefined for the leaf node, and should match against the empty string, not open or close. This fixes the issue where a leaf row, displays an icon in hypergrid

[juliostanley]

Here is the deletion of the property for leaf rows.

https://github.com/openfin/hyper-analytics/blob/master/js/DataSourceTreeview.js#L192