iterative mergesort added

[kgashok]

@OmkarPathak, the iterative mergesort is very helpful to demonstrate how the mergesort algorithm works, in various stages.
In the verbose=True mode, for e.g.

before sorting [4, 0, 9, 8, 2, 5, 3, 1, 6, 7]
[[4], [0], [9], [8], [2], [5], [3], [1], [6], [7]]
[[0, 4], [8, 9], [2, 5], [1, 3], [6, 7]]
[[0, 4, 8, 9], [1, 2, 3, 5], [6, 7]]
[[0, 1, 2, 3, 4, 5, 8, 9], [6, 7]]
after sorting [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]