Add ordered parallel dataset iterator

[anibali]

tnt.ParallelDatasetIterator does not guarantee order. Since there are situations where repeatable experiments are desirable, this PR proposes adding a new tnt.OrderedParallelDatasetIterator.

[lvdmaaten]

This is a great idea!

I'm not sure if I'm happy with having a separate OrderedParallelDatasetIterator though, as it has a lot of copy-pasta from ParallelDatasetIterator. Why not add an optional input argument ordered (with default value false) to the constructor of ParallelDatasetIterator?

[anibali]

That's a fair point, there is a lot of copy-pasted code. I'll look into doing it in your proposed way after the weekend.

[anibali]

@lvdmaaten I've merged the code in with ParallelDatasetIterator. Let me know what you think.

[lvdmaaten]

Looks good, thanks!