Apply Aggregation Without Group By

[DarshanSP19]

Now the aggregation works without a group by clause.
Queries like below are now supported.

SELECT
  MIN(temperature)
FROM 'gs://gcp-public-data-arco-era5/ar/full_37-1h-0p25deg-chunk-1.zarr-v3'
WHERE
  time >= '2023-01-01' AND
  time < '2023-02-01'

[mahrsee1997]

LGTM.