Computing #sum panics when aggregating over non-numeric values

When using `#sum` over exclusively non-numeric values, nemo panics with the following error message:

```
thread 'main' panicked at nemo-physical/src/tabular/operations/aggregate.rs:308:25:
failed to compute aggregate result
```

This is because aggregates are currently implemented using the `TrieScan` interface, which does not allow for missing values.

Since the implementation of `TrieScanAggregate` is already complex enough, I would rather just rework how aggregates are computed using a technique similar to `RowScan`s.

This issue was first discussed here: #629.