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](https://github.com/bilke/ogs/blob/fa36408888cbe5c5475e20c0719b7f132209537c/Applications/CLI/Tests.cmake) for a usage example.
See also a [preliminary documentation](http://docs.opengeosys.org/docs/devguide/testing/test-data).
Somehow diff is looking for files in source directory instead of the binary directory.
Trying to add diff tests I get following from `ctest -VV`:
```
test 3
    Start 3: ogs-GroundWaterFlowProcess-time-diff

3: Test command: /usr/bin/cmake "-Dcase_path=/home/naumov/w/ogs/Tests/Data/Elliptic/quad_20x10_GroundWaterFlow" "-Dcase_name=GroundWaterFlowProcess" "-DTESTER_COMMAND=/usr/bin/diff -sbB /home/naumov/w/ogs/Tests/Data/Elliptic/quad_20x10_GroundWaterFlow/quad_20x10_constMat0_result_expected.vtu /home/naumov/w/ogs/Tests/Data/Elliptic/quad_20x10_GroundWaterFlow/quad_20x10_constMat0_result.vtu" "-DCMAKE_BINARY_DIR=/home/naumov/w/ogs/build" "-P" "/home/naumov/w/ogs/scripts/cmake/test/AddTestTester.cmake" "/home/naumov/w/ogs/build/Tests/Data/Elliptic/quad_20x10_GroundWaterFlow/quad_20x10_constMat0_result_expected.vtu"
3: Test timeout computed to be: 9.99988e+06
3: /usr/bin/diff: /home/naumov/w/ogs/Tests/Data/Elliptic/quad_20x10_GroundWaterFlow/quad_20x10_constMat0_result_expected.vtu: No such file or directory
3: /usr/bin/diff: /home/naumov/w/ogs/Tests/Data/Elliptic/quad_20x10_GroundWaterFlow/quad_20x10_constMat0_result.vtu: No such file or directory
3: CMake Error at /home/naumov/w/ogs/scripts/cmake/test/AddTestTester.cmake:14 (MESSAGE):
3:   Error exit code: 2
3: 
3: 
3/4 Test #3: ogs-GroundWaterFlowProcess-time-diff ...***Failed    0.01 sec
```