Group by multiple features

## Use Case

Requested by @MFarhadloo25! Majid would like to group by multiple features and be able to toggle their visibility in the plot.

For example, Majid wants to group by the experimental condition first, and then by movie, and then by frames. He describes wanting to see a hierarchy of checkboxes in the Grouped download section, where he can choose the levels in the hierarchy and then check/uncheck each group.

## Solution

_Please describe your ideal solution_

## Alternatives

_Please describe any alternatives you've considered, even if you've dismissed them_