Add fine-tuning workgraph with multiple sources of structures

[ElliottKasoar]

In most cases, we'll want to combine results from multiple sources to give us structures to use for fine-tuning, rather than a single calculation type.

E.g. geomopt structures + NEB + EoS (+ filter?), all of which we'll need to do DFT single points for, before using these for fine-tuning