Get rid of `.data_original`

[olgabot]

It's confusing which dataset is stored -- .data or .data_original. It's much simpler to have a single dataframe for the user to itneract with, and then they'll know for sure which one gets saved. Right now, for study.expression, study.expression.data_original gets saved when you do study.save() but study.metadata.data does for metadata... it's confusing and easier to just assume the user has filtered their data exactly how they want already.