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.
Good catch!