initial implementation to display displacement fields as quiver plots

Currently this implementation only works with 2D and does not work
with interactive versions of my show, due to need to access the
current matplotlib axes before it is shown.
fix extraneous statement in myshowDisplacement function
ENH: First test is added

This test needs three input argument: fixed image, moving image and the
baseline image all from the BRAINSTools test data.

It runs 4 stages of registration, and itkTestDriver is used to compare
the output moving resampled image to the baseline image.

Many function definitions that are common between different tests are
put in one file (TestFilesCommonCode.ipynb), and this file is run at the
beginning of each test using ipython magic functions.