Add metadata for DIII-D physics variables

[nbarbour13]

Added metadata for DIII-D physics variables to disruption_py/machine/d3d/config.toml for Issue #581 .

• Missing descriptions: n1rms, n1rms_normalized.

  • Explicit descriptions for the n1rms field were missing from the trees I searched. Instead of making an inference, I will defer to what comes out of the PR review discussion.

• Out of scope:

  • Unified variable names across machines. This may be a breaking change for downstream users, so it may be worth discussing for a later release/PR.
  • Metadata for generic machine physics methods. Adding support for a new config.toml for the generic machine physics methods is left to a future PR.
  • Updated metadata for C-Mod physics methods.

[Copilot]

Pull request overview

Adds/extends the DIII-D machine configuration to include metadata (description, IMAS path, units, and optional validity ranges) for many DIII-D physics output variables, aligning with the metadata approach already used for other machines (e.g., C-Mod) and supporting Issue #581.

Changes:

• Adds a large set of [d3d.physics.attributes.*] metadata entries (descriptions, units, IMAS mappings, and validity ranges where known).
• Preserves existing DIII-D physics/test configuration while expanding attribute metadata coverage.

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[gtrevisan]

@zapatace could you help double-check this IMAS mapping?
otherwise LGTM.

• you are right that some renaming might be warranted, possibly by taking the last qualifying name of the IMAS structure -- eg: aminor --> minor_radius.
  we should discuss this at one of the next meetings.
  EDIT: what about IMAS last name as z? maybe current_centre_z?

• generic physics methods should definitely get attributes, too!
  good catch, let's tackle that in a separate PR.

• if you caught any C-MOD inaccuracy, feel free to push changes.
  if easier, we could also do one big metadata PR, rather than having three (eg: C-MOD, DIII-D, and generic)...