MRG: test and extend PARREC array proxy

I forgot to add the array proxy test for PARRECArrayProxy.

To make the tests pass, add an interface to return slices from the proxied
array.

Optimize this a little using `fileslice` and some fancy broadcasting tricks.
On the other hand, this fancy code makes it more urgent we have an image with per-slice scaling, to test this with.  Our test image has identical scaling for each slice.