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.
I was going to ask why you actually needed to construct something of the proper shape instead of just using broadcasting... I see it's because it makes this step much easier. (Otherwise you'd have to figure out which dimensions you shouldn't slice, and which you should.) Neat!