ENH: Improve get_data for 4D+ multiframe

Use pixel_array instead of PixelData so pydicom handles many
complexities. Sorts/reshapes the array in place.
BF+TST: Fixed lookups for iop/ipp/pixspacing
TST: Add test data for previous commit
TST: Point multiframe test at correct file.
TST: Add test that makes sure resulting data order doesn't change for 4D multiframe.
CMP: Fix Python3 incompatability in last commit.
Merge pull request #1 from moloney/enh-multiframe-rebase

ENH: Improve get_data for 4D+ multiframe

Fixes from Brendan