As of this commit the transformix filter executes correctly on
call to GetOutput(), but calling GetOutput on the elastix filter
does not trigger an Update() as it should

Previously, separate containers was used for fixed and moving images.
The ITK pipeline would be driven only by the first fixed and first moving images.
This commit introduces extra bookeeping so that all images can be added to the
ITK input container and split into seperate containers for elastix at a later
stage. This is necessary because only ITK input container is part of the
pipeline mechanism.

Another (and possibly simpler) solution would be override the Update() function
and check that all image containers are up to date. However, this places the
responsilibity of pipeline mechanism maintenance on us. It is probably better
to keep this upstream.

TODO: Test fails because of problems with setting up multiple instances
of xout in the same process

TFixedImage as the ImageSource template since the output image is
always resampled to the fixed image domain.

TODO: Fix 'error while setting up xout' error when running multiple
ElastixFilters in the same binary (see ELASTIX-56 in SuperElastix Jira)

Adds a stand-alone ITK filter that wraps the elastix library
in a fully pipeline-compatible process object. The original
library interface was deprecated in favor of direct use of
elastix internals in the filter's GenerateData() for more
control over pipeline inputs and outputs.

TODO: Support multiple fixed and moving images
TODO: Return TransformParameters

WIP: Segmentation fault when trying to access result image

This build fails because VectorContainer has a private default
constructor