Add `MOM.DBW.SAW.Q_Result.slice`
Add support for aggregation functions to SAW.QX

* Allow `Q.SUM(...)` as operand of expressions

  - Factor `TFL.Q_Exp._derive_expr_class`;
    call it for `_Call_` and `_Sum_`, too

  - Factor `TFL.Filter._Filter_S_._add_derived_classes`;
    add derived classes for all entries in
    `TFL.Q_Exp.Base.expr_class_names`.

* Add support for `Q.AVG`, `Q.COUNT`, `Q.MAX`, and `Q.MIN`

  - Factor `TFL.Q_Exp._Sum_` to `TFL.Q_Exp._Aggr_`

  - Factor `TFL.Q_Exp._derive_aggr_class`

  - Factor `MOM.DBW.SAW.QX._Sum_` to `._Aggr_`

* Use `coalesce` and `label` in `MOM.DBW.SAW.QX._Aggr_.apply`