RF: use assert_equal, assert_true for assert

assert gets optimized out by the -O flag, but assert_true etc do not.
Then, running the tests with the -O flag would quietly skip the assert
tests.
BF: note some multiframe tests as dicom tests

Multiframe data and affine tests require pydicom; decorate for this.
RF+TST: refactor multiframe image_shape with tests

Add a docstring to image_shape for multiframe.  Refactor for my own
understanding.  Add tests with fake dicom data.