BF: note dependency on pydicom >= 0.9.7

Appears to be needed for correct reading of names of DICOM fields in
Philips wrapper.
RF: move multiframe tests to own test class

Might be better for encapsulating these tests, make it easier to test
subclasses.
RF: generalize fake_frames method

Make general enough to use for other properties.
TST: add tests for multiframe voxel sizes

Refactor voxel_sizes function to raise errors for missing values rather
than returning None.