Support for DataStructureDefinition (DSD)

[Tungurahua]

This issue creates a reference number for the creation of a DSD parser which can be used for naming an issue branch for the topic.
I'd sugggest to make this a function that creates a list of dataframes for every codelist contained in the DSD file, wherein each dataframe has a code and description column.

At a later iteration this object can be assigned to a slot of an S4 object representing the DSD (could be named SDMXdsd, inheriting from the SDMX class) which in itself could then be used to create a class that compounds the header information, the codelists and the actual data as dataframe. However, these thoughts should probably be detailed in another issue.

[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...