Allowing IQL to loop on columns

Details given in this [Notion card](https://www.notion.so/chi-mit/Iterating-over-columns-in-IQL-bb3c612206f948879be9bef91f8dcc02?pvs=4).

TLDR: This can be useful for computing MI between all pairs of columns, or filtering out columns which are probably independent from others, as in Example 3 in Figure 3 in [this paper](https://proceedings.mlr.press/v54/saad17a/saad17a.pdf).