Extend SDMX-ML writer for specimen collection

[khaeru]

Development of dsss highlights classes that appear in the specimen collection (sdmx-test-data), can be read, but cannot yet be written to SDMX-ML 3.0 or 2.1.

Extend .writer.xml to cover:

• common.ProvisionAgreement
  • UNICEF:GLOBAL_DATAFLOW_UNICEF_UNICEF(1.0)
• v21.DataSet with group keys
  • StructureSpecificDataSet-ECB:ECB_EXR_RG-8db23608ea68221c
  • StructureSpecificTimeSeriesDataSet-ECB:ECB_EXR_CG-edb6d92396a6dc84
  • StructureSpecificDataSet-ECB:ECB_EXR_SG-f6e62e6c189152c1
  • GenericTimeSeriesDataSet-ECB:ECB_EXR_RG-8184fb683bf2894a
  • StructureSpecificDataSet-ECB:ECB_EXR_SG-491d1fc56da31890
  • StructureSpecificDataSet-ECB:ECB_EXR_SG-5da95bdbfc876a50
  • GenericDataSet-ECB:ECB_EXR_SG-dd99a8462c9a1019
  • GenericDataSet-ECB:ECB_EXR_RG-7e6eed50954da809
  • GenericDataSet-ECB:ECB_EXR_RG-6f216eb64a75b317
  • StructureSpecificDataSet-ECB:ECB_EXR_RG-1d504305ab21fb2f
  • StructureSpecificTimeSeriesDataSet-ECB:ECB_EXR_SG-93163e9579f23c5d
  • GenericDataSet-ECB:ECB_EXR_SG-aba60ad78972b953
  • GenericDataSet-ECB:ECB_EXR_RG-fa0236894e9f33f5
  • GenericTimeSeriesDataSet-ECB:ECB_EXR_SG-1fdd1c8fac668340
  • GenericDataSet-ECB:ECB_EXR_SG-1ebef11c4e8ef9d7
  • StructureSpecificDataSet-ECB:ECB_EXR_RG-bdf99ebf4b9c9fc6
• v30.DataConstraint
  • EXAMPLE:ISIC_CONSTRAINT(1.0)
  • EXAMPLE:NACE_CONSTRAINT(1.0)
• v30.Dataflow
  • EXAMPLE:LABOUR_ISIC(1.0)
  • EXAMPLE:LABOUR_NACE(1.0)
• v30.DataStructureDefinition
  • EXAMPLE:DSD_GEO_EXAMPLE(1.0)
  • ECB:ECB_EXR(1.0)
  • EXAMPLE:LABOUR(1.0)
  • ECB:ECB_EXR_COMPLEX_ATTRIBUTES(1.0)
• v30.GeoGridCodelist
  • EXAMPLE:CL_GRID_LOCATION(1.0)
• v30.GeographicCodelist
  • EXAMPLE:CL_ADMIN_AREA(1.0)

cf. #87, #85.