Grouping aggregation with expresssions

Grouping aggregations with expresssions are currently not allowed and you have to use sub-selects to workaround it.

Example that does not work
```sql
select name,avg(cast(system as int)) from system_tables group by name order by name
```
and workaround
```sql
select name,avg(sys) from (select name,cast(system as int) as sys from system_tables)
```
