OPTIMADE v1.3 release planning

[ml-evs]

Logistics:

• Tidy up final PRs
• Release candidate 1.3.0-rc.1
• Changelog
• Implementation period (deciding whether to wait until one working trajectories implementation exists before releasing)
• Release

Spec development to-do (from @rartino):

• Make sure symmetry is in place (e.g., Introduce wyckoff_positions property #570)
• Make sure there are property defs for everything in the spec (and maybe remove the definitions from the spec?)
• Trajectory fields

[merkys]

Just FYI, addition of missing property definitions is being tracked in #555 and #563.

[esoteric-ephemera]

Regarding trajectories: Materials Project has been developing atomistic/electronic-structure schemas to better handle simulation output, and to support cloud storage tech like parquet in addition to JSON

From a quick look at #377, #481, our trajectory schemas appear to be compatible with the proposed OPTIMADE ones, with some migration

Still, if there's a chance to offer feedback on how trajectory data could be structured, happy to discuss this. Tagging @tschaume and @tsmathis in case they want to chime in