Sum does not support summing of aggregates

[AndrewSisley]

Child of #28

Is a bit of a pain that I forgot about this, as it prevents the surfacing of inner aggregates to the top of query.

For example (test and very partial implementation in branch sisley/feat/I149-sum-of-sums):

                                query {
					users(groupBy: [Name]) {
						Name
						_sum(field: {_group: _sum})
						_group (groupBy: [Verified]){
							Verified
							_sum(field: {_group: Age})
						}
					}
				}