[Backend] Automatically detect changes in the data (and update those)

[FStriewski]

Description (user story)

We only want to run preprocessing (again) on updated data, not the full data set (every time). For this we need to check which rows have been updated and which haven`t.

Considerations:

• Hash data to check if the (whole) dataset has changed at all
• Find out how to rerun processing on changed data