Use external data series format to support alternative file versions
Use external data series format to support alternative file versions

Most test have a baseline named file.nrrd, some tests have alternative
baselines named file.1.nrrd. The ExternalData module can support
downloading these alternative images as part of it's series
feature. Here all the generated test use the ",:" series format so
that if an alternative base exists then it will be downloaded.

kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix