Add option to find unique elements in nested sequences when calling `Dataset.unique`
Hi @mariosasko!

Has this been patched into any of the releases? Hi! Not yet, would you be interested in contributing a PR? I can give you some pointers if needed. 
It would be nice to have an option to flatten nested sequences to find unique elements stored in them when calling `Dataset.unique`. ~~Currently, `Dataset.unique` only supports finding unique sequences and not unique elements in that situation.~~