QoL symmetry: `Run.inputs` mirroring `Run.outputs`

[liamhuber]

Because I'm discovering that sometimes I want a simple label:value dict for the inputs as well as the outputs and for basically the same reasons. Run.outputs is just a property shortcut, so this is an easy symmetry to add.

[liamhuber]

Ah, shoot. Actually, at this exact moment such an improvement belongs over in pyiron_workflow. I'm already operating in a post- #310 mindset.