Use CMake ExternalData command to download data for automatic logic tests

[cpinter]

This is a relatively new CMake feature that would be nice to use. This will reduce source code size and simplify repository.

Migrated from https://app.assembla.com/spaces/slicerrt/tickets/694-use-cmake-externaldata-command-to-download-data-for-automatic-logic-tests/details

[cpinter]

Using ExternalData for many files has a big overhead (re-generate MD5 sums and update in CMake files), so for DICOM data it would make sense to try to upload them as zip files and then extract it from the test program. Downloading big files may not be reliable on some internet connections, so caution must be used.