Task #3651: Add skip field to Mapfile

- Implement a mapfile replacement: DataMap adding a skip field to the host, file pair.
msss_target_pipeline and msss_calibrator_pipeline do now implement the skip behaviour for dataproduct.
msss_imaging_pipeline use the new DataMap and MultiDataMap classes for dataproduct communication. Does NOT listen to the skip field (default behaviour)
- Some minor changes in the location of imports (from start of module to function) to allow running of unittest on windows
- Update of the unittest to reflect the new behaviour and adding of DataMap unittests