WIP: adding SimpleFilters as internal modules via externalproject

Following the example of MultiVolumeImporter a new external project
was added to download the SimpleFilters repository, the source
directory is propagated into the Slicer project and it is added as a
sub-directory to the build in scripted modules.

TODO: Moved the SimpleFilter repo to the SimpleITK group, using filter
branch to remove the jsons, then use a git merge strategy to maintain
files.