Aggregation

[lars-reimann]

Is your feature request related to a problem?

Besides the explicit methods of Column, we do not support aggregation yet.

Desired solution

• New subclass of Cell: AggregatedCell or AggregationCell.
• Think of it as a Cell with memory.
• New Table method aggregate
  • Callback (Row) -> AggregatedCell
  • Returns a new Table with a single row
• For symmetry, also add an aggregate method to Column
• Other mapping Table methods (e.g. transform_column) should also be able to handle AggregatedCell, but produce the same row count as a normal Cell, e.g. by filling the entire column with the same computed value.
• Operations that mix AggregateCell and Cell should produce a Cell, so they cannot be used with Table.aggregate. This would otherwise be ambiguous, as we want a table with a single row.
• Namespace Cell.agg that wraps polars's aggregation expressions.

Possible alternatives (optional)

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Additional Context (optional)

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