Adapt sumMap to accept an array of strings as the key column. This is useful when we do not want or cannot make the keys be numbers.
Signed-off-by: Baudouin Giard <bgiard@bloomberg.net>
It used to be that sumMap would return the same type as the values
columns. If columns of Array(UInt8) were to be given, that would really
easily cause oveflow. It now uses `getWidenDataType` (and ultimately
`NearestFieldType`) in order to define the result type.
