Add sidepanel options for hierarchical

[keckelt]

• Release number or git hash: 2.0 in kuk ordino
• Web browser version and OS: chrome 62 @ fedora 25

Add some options for hierarchical columns in the sidepanel

[mstreit]

• Histogram (of currently set cut-off?)
• Missing values filter

[thinkh]

@keckelt Is this feature request still valid? If yes, how could such a filter option look like? What column type did you use?

[keckelt]

The hierarchical attribute in my dataset is displayed as categorical.

I can specify a cutoff in the column header, but the sidepanel (in Ordino actually) only displays the attribute name:

I would add a control to select some subtree, which then removes the items that are not part of that subtree from lineup.
Example: https://neofusion.github.io/hierarchy-select/