Save affine and ffd transformation to file

[enricotagliavini]

Save affine and ffd transformation to file so we can load them later when needed (e.g. for a fractal task to compute transformed images)

For the affine transformation we can use numpy.save() and numpy.load() see https://numpy.org/doc/stable/reference/generated/numpy.save.html

For the ffd transform we need to use SimpleITK WriteTransform() and ReadTransform() functions, see https://simpleitk.readthedocs.io/en/master/IO.html