Union operator

[hadrienk]

Implement union (records)

Syntax
Dataset ds_2 := union (Dataset ds_1 {, Dataset<?> ds_1}*)

Test:

• Test antlr grammar
• Create antlr grammar
• Test that all the Datasets ds_1 must have the same IdentifierComponents and MeasureComponents, in name and type (static).
• If only one argument is given, it is returned unchanged.
• Test that in the output there cannot be two rows with the same values for all the IdentifierComponents and MeasureComponents.
• Test Attribute propagation
• Implement a Visitor in the script