The grouping clause "group all" allows only one expression

[hadrienk]

Issue Description

The group all can be used to group on an expression instead of identifier. The specification defines it as:

converts the values of an Identifier Component using conversionExpr and keeps all the resulting Identifiers

The "resulting identifiers" in plural is misleading. The grammar only defines one expression:

groupingClause:
    GROUP op=(BY | EXCEPT) componentID (COMMA componentID)*     # groupByOrExcept
    | GROUP ALL expr                                            # groupAll
  ;

There's also no way to specify the name of the resulting components.

Proposed Solution

Change the spec and grammar so that this statement is valid:

test := ds[aggr foo := sum(bar) group all id1 + id2 as id1_id2, length(id3) as id_length];