add a way to "load" a file displayed in the explorer as a variable

[gdementen]

• always as Array?
• theoretically, we could write a generic loader using the adapters which return a 2D Array (most likely of object dtype)
  • each label column would become one label in the second axis
  • while this would be nifty, I am unsure it would be that useful in practice
• for smallish dataset we could compute the cartesian_product of labels
  • this would be a lot more useful but absolutely requires a check to avoid trying to load structure where the cartesian product would not fit in memory (e.g. large dataframe-with-multi-index structures).
• what we really need is a way to load the formats we wrote ourselves/which can be loaded currently (.h5, directory of .csv and .xlsx)