Computing #sum panics when aggregating over non-numeric values

[aannleax]

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 RowScans.

This issue was first discussed here: #629.