padded blocks

This PR implements two main changes:
* it introduces a `PaddedBlocks` class, which acts like the existing `SimpleBlocks` but for an additional padding region of redundant voxels around the edge of each block
* when a tuple of int is passed into a Simple (or Padded) BlockingStrategy to explicitly specify the way in which to divide up an image, this tuple will now by default be interpreted as a block size in pixels, rather than a number of splits to be applied to that dimension of the image. The splitting behavior can be maintained by passing `units="splits"` as an additional argument to `toBlocks` or any of the family of methods that end up calling `toBlocks`.

I'll attach in comments a demo script showing usage of the new padded blocks, to be used with thunder-submit.