Handle deleting data

[shankari]

Suggestion from @PatGendre as part of #364
Also required by GDPR as per @jf87

This is likely to be complex, because the current assumption is that input data is strictly read-only, which is a nice assumption to make wrt reproducibility. I do think that there has been some work done on mostly read-only data, where you can detect the parts that have been modified and recompute all results accordingly, but would have to find it.

Opening this new issue for discussion since this is likely to be a pretty tricky change.