Method to find entity combinations "missing" from dataset

Most of the new filtering apis (#209) focus on removing any entity combinations that are missing in one or more components. For example, component B is missing subject 2, so subject 2 is completely excluded from `.expand()`.

It would be helpful for QC to have a method to print all of these missing entities. This would let developers and users quickly query missing parts of their datasets. Technically, this is equal to:
```py
# pseudocode
product(*dataset.entities.values()) - dataset.zip_lists
```

In other words, the maximal zip list subtracted by the actual zip list.

My idea is to have a base method that returns a `zip_list` like representation of all missing groupings, and possibly another convenience method to print the list in a nice table. I need to think about the exact API yet, but if anyone has ideas please share!

Review the new filtering APIs in issue #209 and the dataset representation described here, especially entities and zip_lists. Clarify the base method's return shape and whether the convenience table method is in scope; done means missing entity combinations can be queried without being removed by filtering.

Written by the indexing model from the issue text.