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.
DOC: reformatting comments, fix a typo
TST+RF: test multiframe scale data

Refactor _scale_data method to use RescaleSlope, RescaleInter from main
DICOM data if missing from frame data.
RF: move fake_frames helper out of test class

Ready for some refactoring of tests to use helper in test class init.
RF+TST : add tests for fake data

Simplify get_data method a little