Add support for grouping column in hep-explorer

[jwildfire]

Per SafetyGraphics/safetyGraphics#662. Relevant in some other renderers as well.

[jwildfire]

See #100 and associated PR for an implementation outside of safetyGraphics

[jwildfire]

Fix is pretty straightforward, just need to make a custom init that merges in the treatment column from dm and maps it to settings.group_col.

Would be nice to avoid having to do this for every chart though ... Maybe safetyGraphics could provide standardized access to certain key mappings (group_col, filters, etc). Will think about if there's a straightforward way to implement that ...

[jwildfire]

@samussiah I wonder if it would be easy to add a helper function that does this for multiple charts in the new modular htmlwidget framework you've set up.