Merge datasets

The user wants to be able to visualise population datasets from different scientific experiments (i.e. coming from different data files) into the same experiment in the app. I am assuming there are a number of ways in which we can solve this, one would be to allow loading multiple files during the experiment creation phase, another would be allowing to 'add' populations to an existing experiment. I would leave the latter on hold for now and focus on allowing the user to load multiple files that get merged in the same experiment. @stephenlenzi already solved this manually I guess, could you please elaborate on this?

Point 4b from here: https://docs.google.com/document/d/1iAqMKX8dr7YVa1dsDFFzxOl2Wd89fngi/edit#