Expand Information Model coverage

[khaeru]

sdmx1 currently covers all widely-used features from the SDMX 2.1 IM §§1–6 and §10 (with some exceptions; see below). Other parts of the IM are uncommon or not visible anywhere in “the wild,” but could still be implemented if there is desire/resources.

This is a master issue for discussing these and tracking other issues and PRs.

• As of v2.6.0, bold and “some examples exist” refer to cases where one or more sources currently return actual messages that cannot be parsed by code (as of version 2.6.0). These are excluded from tests here:
  

  sdmx/sdmx/tests/test_sources.py

  Lines 32 to 38 in 300a388

  	#: Failures affecting **all** data sources, internal to :mod:`sdmx`.
  	xfail_common = {
  	"contentconstraint": XMLParseError, # KeyError
  	# <str:StructureSet> <str:HierarchicalCodelists> not implemented
  	"structure": XMLParseError,
  	"structureset": XMLParseError, # <str:StructureSet> not implemented
  	}

• As of v2.13.0, all of these exclusions have been removed.

§7 Metadata structure definition and metadata structure set.

• Implement MetadataSet, MetadataStructureDefinition, etc. #73.
• Update ISTAT service URL #105 added MetadataflowDefinition, MetadataStructureDefinition.

§8 Hierarchical code list. The base Codelist is already hierarchical; the chief difference is this one seems to support multiple hierarchy.

• Some examples exist.
• Classes: HierarchicalCodelist, HierarchicalCode, Level, Hierarchy, CodingFormat.
• Add metadata structures and sets to IM and SDMX-ML reader #152.

§9 Structure set and mappings.

• Some examples exist.
• Base classes: StructureSet, StructureMap, ComponentMap.
• Individual classes: ConceptSchemeMap, CategorySchemeMap, CodelistMap, OrganisationSchemeMap, ReportingTaxonomyMap, ConceptMap, CategoryMap, CodeMap, OrganisationMap, ReportingCategoryMap.
• Add metadata structures and sets to IM and SDMX-ML reader #152 (partial)

§10 Constraints. All visible usage is covered, but some imaginable applications are not.

§11 Data provisioning. Partly covered. Some of these classes map very neatly to existing functionality in SDMX, e.g. RESTDataSource could be very similar to Source. However, they are somewhat underdefined; e.g. the IM does not specify property names for all the information that has—in practice, in this package—proven necessary to track actual behaviour of sources, etc.

§12 Process and §13 Transformation and expressions. These sections appear useful, but there are no (?) real-world examples and specimens to check behaviour against, so implementation is probably premature.