Group items into new harmonised variables

## Description

can we add to the export, groups of similar items? E.g. everything to do with height across 5 studies? Perhaps this could also be another view in the visualisation in the tool.

See mockup:

https://github.com/harmonydata/hackathon/blob/main/find_variable.png

## Rationale

Users have requested this feature. Because there is otherwise a manual step going from the similarity matrix (which is currently in the export ) to harmonised variables.