RF+TST: test PARREC array proxy API; add slicing

Add `__getitem__` placeholder method for PARRECArrayProxy, to fulfill
required API, and test with API validators.
NF: add strided_scalar function

Function to return scalar broadcast out to form full array of given
shape.
NF: implement volume by volume reading for PARREC

Use fileslice when we can to do volume-by-volume reading.
TST: PARREC image with varying slope / intercept

Test with ArrayProxy API.
Merge pull request #269 from matthew-brett/parrec-arrayproxy-api

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.