Revisit and document structure inputs/outputs

[ElliottKasoar]

It would be useful to do a pass over the input/output specifications for calculations, and make sure how this works is clearly documented, and consistent between calculations.

For example:

• Structure inputs are usually (always?) include StructureData, but structure outputs are often, but not always, SinglefileData
• In some cases e.g. the trajectory from geomopt, we store both: https://github.com/stfc/aiida-mlip/blob/main/aiida_mlip/calculations/geomopt.py#L92:L93 / https://github.com/stfc/aiida-mlip/blob/main/aiida_mlip/parsers/opt_parser.py#L82:L87
  • Do we want a similar duplication for all structure outputs?
  • If not, we may want a helper function that converts structures in SinglefileData back into StructureData, e.g. for use in Add descriptors filter workgraph #185 and other workflows where we chain together calculations