Update signature of Collect to take an Operation.

Collect is updated to take an operation that returns a sequence of ops, instead
of taking a sequence of ops directly.

CollectArray is deprecated in favor of using logic.Collect directly.
Update signature of Mean and Variety to take Seq[Raster].

Mean and Variety now take Seq[Raster] instead of Array[Raster], and no longer
use the depreciated CollectArray.
Merge pull request #536 from joshmarcus/topic/collect

Update signature of Collect to take an Operation.