ENH: Direct initialization for SyN registration process

This patch allows us to save the last state of the current registration to
the disk using the "save-state" option; then, it is possible to use the
"restore-state" option to load and restore the saved state later for the
direct initialization of the next registration process.

This patch should be built based on the following changes to ITK:
http://review.source.kitware.com/#/c/17729/