Retrieve data from multiple schema versions

[apdavison]

Where we have data using older schemas, we should retrieve it if possible (maybe have a list of supported versions, we create only using the most recent, but read from/update any)

[apdavison]

Implemented in #118 (milestone 0.15): fairgraph now provides both openMINDS v4 and v5 class sets, selected per client.

client = KGClient(host=..., openminds_version="v5")   # "v4" is the default
import fairgraph.openminds.v5.core as omcore

Existing code is unaffected — fairgraph.openminds.<domain> still resolves to v4.

This differs from what was proposed here in 2019. Rather than fairgraph reading any version transparently, the version is chosen explicitly per client: a v4 class and its v5 counterpart share the same @type URI in the KG, so a response cannot be assigned to a version by inspecting it — something has to say which to use. Writing is likewise per-client rather than always using the newest schema.

v5 support is experimental until the KG migration to v5 completes; see the 0.15.0 release notes.

The older schemas this issue originally had in mind were v3, which the KG no longer serves — the v3/v4 adapters were removed in #112.