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.
:) - yes - I was just starting on something like that, when I realized that I could probably get numpy to work it out for me...