Saving scfres to JSON should pretty-print

[Technici4n]

The call in AiidaDFTK: save_scfres("self_consistent_field.json", scfres; save_ψ=false, save_ρ=false). This does not pretty-print the output which makes it unreadable by human eyes. Note that pretty-printing might increase the file size.

[mfherbst]

File size has been my main reason for not doing it. If you show its not too big a deal we can think about it.