[TDF] Better interplay between TDFActions and Operations

This extensive refactoring standardizes and simplifies the way `TDataFrameActions`
and `Operations` work together.

- logic should be clearer
- avoid the creation of an intermediate shared pointer and lambda function
  that were passed to the `TDFAction`
- `TDFActions` are now templated on the `Operation` type rather than on the
  type of a callable, possibly reducing the total number of template
  specializations of large applications
- `TDFAction` now directly stores the relevant operation as data member
- `TDFAction::Run` calls `Operation::Exec`
- `~TDFaction` calls `Operation::Finalize`
- `ForeachSlot` is now implemented in terms of an `Operation`
  like all other actions