Use a trailing slash to reference a directory. Require that a list
of associated files be specified to select from within the directory.
One may simply use DATA{Dir/,REGEX:.*} to reference all files but
get a directory passed on the command line.
Refactor use of the ExternalData_SERIES_MATCH value to avoid assuming
that it has no ()-groups that interfere with group indexing.
Extend the Module.ExternalData test to cover this case.
Add a Module.ExternalData test to verify data retrieval and test
argument DATA{} references.
Add a RunCMake.ExternalData test to verify error handling and automatic
transformation of a raw data to a content link and staged object.
Brad King