Tempo causes collect() deprecation warning when used in DLT pipeline

[BradLotsberg]

There are a number of collect()[0][0] instances in the Tempo code which trigger deprecation warnings if used in DLT pipelines. Perhaps replace collect()[0][0] with head()[0]. Accomplishes the same thing while avoiding the deprecation warning and might even bring a marginal performance boost since only the top row instead of the whole data frame would move to the driver node.

[tnixon]

This is a good suggestion, thanks @BradLotsberg.
@yuriymargulis-db - perhaps you can pick this one up?