Teach Utilities/Maintenance/SourceTarball.bash to generate separate
archives for the source code, normal data, and large data.  Organize
the archive layout so that they can all be extracted on top of one
another.  Make the data archives extract into <prefix>/.ExternalData
to appear as if the content came with the source tree.

Add .ExternalData/README.rst to document the directory and indicate its
existence.  Change the BUILD_TESTING default to OFF when the file is not
present.  Teach SourceTarball.bash to leave this file out of the source
tarball and put it in the normal data tarball.  With this approach
users can download and build the source with or without the data and
never hit the network for test data by default.

Change-Id: I33007959c87ea60a9b11f49e5f7af351fc2ca9b7