Improvements for testing with external data

This will be useful once the groundwater flow process outputs something ...

Changes:

• flexible AddTest() parameter parsing
• added parameter for
  • executable arguments
  • referenced external data files
  • diff data files
• gives fatal errors when a requirement for a specific wrapper or tester is not met
• added documentation
• added target sync-data (on enfinv1 only) which syncs your local data-directory to opengeosys.org

See Applications/CLI/Tests.cmake for a usage example. See also a preliminary documentation.