Add support for calculated fields that can return tables (user-defined functions)

Especially for custom calendars it would be awesome to define your own functions for YTD, etc. But more generically being able to define a measure that returns a table/filter would be awesome. [LargeNationalCustomers] := FILTER(Customers, Customers[Region] = "National" && Customers[NumEmployees] > 100) [Revenue_LargeNatl] := CALCULATE(SUM(Transactions[Revenue], [LargeNationalCustomers]) The decision to limit calculated fields to just scalars... feels almost arbitrary and counter productive.