`Image.distribute()` - enable multi-cell input

[demisjohn]

Enable Image.distribute() to take more than one Cell [C,R] at a time. This would allow direct passing of get_ValidCells() like so

MyImage.distribute( MyJob.get_ValidCells(), multi=True) to distribute to all valid cells. (Or automatically check if the iterable is >2 dimensions deep, so no need to multi=True.)