new Images and Series unit tests

Mostly Just A Bunch of Tests.

Two small bug fixes in here uncovered during testing:
* The images `subsample()` method had an off-by-one error in the dims attribute of the returned Images object when the subsampling factor did not evenly divide the image dimension.
* The images `subtract()` method did not work when numpy arrays were passed, due to a bad equality test between a Dimensions object and a numpy shape tuple.
I do vaguely remember discussing this... as i recall, there's a pretty definite efficiency gain to be had in `pack(sorting=True)`, and possibly also a correctness problem, as you describe here - but so far as I know we still have to validate that there is indeed a correctness issue in `pack`? Regardless, if there is an issue, it's in `pack` and not `maxProject`, yes? 

I can say that *without* the `sorting=True` param, projecting along axis 1 returns values that are out of order, which makes some intuitive sense to me (not having worked through the logic in detail). I'd buy that there may still be some issue with the sorting, perhaps one that only shows up at larger scales / in actual cluster operation? The unit tests of course are run locally, maybe that's obscuring a deeper problem...

Anyway, so far as I'm aware, `maxProject` itself seems to be doing the right thing, at least so far as these rather trivial examples are concerned. How about if we try to replicate the possible issue with `pack` and open up another issue for that if it can be verified?