Support for DataStructureDefinition (DSD) about rsdmx

[eblondel]

I've yet some codes for parsing a codelist, and list of codelist through dsd parsers, coming from the initial package...I have to check these codes, then integrate them into the new S4 package architecture, but i will proceed step by step, starting with single codelists, and then datastructure handling multiple codelists.

[Tungurahua]

A modular approach makes perfect sense. I have been working in the same direction. For me right now this is an exercise to get "up to speed" with the XML package. After that I will have to get "up to speed" with the S4paradigm. All of which will take time, and there won't be tons of half-baked pull requests.

[eblondel]

Hi, i've worked a bit on refactoring the model, adding the base "structure" type. #23 and #24 prepare the S4 basis and other stuff for implementing more parsers, including DSD & codelists...