Add support for Groups in SDMX-ML 2.1 Data readers and writers

[javihern98]

Add support for GroupDimensions in Schema and data readers and writers

[javihern98]

@sosna @stratosn We are planning to support as well the Groups in data messages on SDMX-ML 2.1. We are currently struggling to see how we can include the Group class in the model, as it cannot be a Component class but somehow we must add this information to the Schema (and DataFlow, DataStructureDefinition, etc).

[sosna]

Thanks, @javihern98, and happy new year :).

Is this something that can be discussed after the 1.0.0 release?

[javihern98]

Sure! We can implement it afterwards, just to have it in the backlog for future releases