Tk 2.  Addresses data corruption with mixed slice timesteps

The previous caused requests for data *before* any slices we had to use the
*most* precise data slot, rather than the least precise data slot.

This approach forgets entirely about the available slices, and relies purely
on the retention information (if present) to determine the precision to use.

It is *worse* that the previous solution for the case where no retentions are
specified, but there are mixed precision slices present (but this should never
be a real world case)