Bidirectional OO-LD <-> NeXus (NXDL) converter

[SimonStier]

Convert between OO-LD schemas and NeXus application definitions (NXDL).

Spec-side siblings: OO-LD/oold-schema#109 (dataset storage model, instance-level data references and data bundling), OO-LD/oold-schema#107 (QuantityValue as a base building block) and OO-LD/oold-schema#127 (transport for large tabular instances).

cc @hampusnasstrom

Mapping

• NXDL groups to nested object schemas, fields to typed properties, attributes to properties or annotations
• NX base class and application definition names to the type IRI in the @context
• minOccurs and maxOccurs to required and array bounds
• Field units to the OO-LD quantity value representation

Expressible with standard keywords: group nesting, field names, scalar types, occurrence bounds
and required or optional status. Needing an x- vendor annotation: the NX class identity and the
base class versus application definition distinction, the per-field units attribute if
OO-LD/oold-schema#107 does not cover it, the dimension names carried in NXDL dimensions, and the
link and target semantics of NXDL link elements.

Open questions

• NXDL describes an HDF5 file layout. Decide whether the converter targets the file structure or the logical content, since the two differ for links and external data
• How large array data is referenced rather than inlined, see Scientific profile: dataset storage model (datasets, distributions, storage modes); core ask: instance-level data references (data bundling) oold-schema#109 and Transport / compression: evaluate CBOR (record structures / Packed) for large tabular instances oold-schema#127
• Whether base classes and application definitions are handled by the same code path
• Which NXDL version and whether the FAIRmat extensions are in scope