Task SSB-42: Added az comment about the test data

The test data is big.  Really big.  Too big to store it in a
source code repository.  This poses the problem that the test
has to either download it or to rely on the data being in a
defined location that is always on and always accessible from
everywhere.  Both solutions don't work in all cases.

Bottom line:  we have to figure this out!  For the time being the
test will just fail when the data is not in the expected location.

CAL/CalibrationCommon/test/t_holography_dataset_class.py

@@ -7,6 +7,8 @@ from lofar.calibration.common.datacontainers import HolographyObservation
 
 
 logger = logging.getLogger('t_holography_dataset_class')
+# READ doc/Holography_Data_Set.md!  It contains the location from which the
+# test data must be downloaded.
 path_to_test_data = '/data/test/HolographyObservation'
 path_to_test_data = '/home/thomas/workspace.astron/data/holography'