Add support for column aggregates

[JerichoCross]

If a column is defined as 'numeric', there should also be an option to specify an aggregate (Sum, Median, Mean etc. or leave null for not required). If one of the columns has one of these defined, add an extra row at the bottom with the appropriate aggregates calculated e.g.

DataTable2(
  columns: [
    DataColumn2(
      label: Text("Name"),
    ).
    DataColumn2(
      label: Text("Age"),
      numeric: true,
      aggregate: ColumnAggregate.mean,
    ),
  ],
),

Would produce:

Name	Age
John Smith	28
Steve Jones	31
David Cross	22
	27

[maxim-saplin]

Thanks for the suggestion. No plans to work on that feature.

This can be easily achieved via custom row. DataTable2 is not intended to be a rich control in traditional sense, it is a think widget with little behind the scenes logic.