feat: add `get_saved_state_idxs`, handle problems with no system in SavedSubsystem constructor#834

ChrisRackauckas merged 2 commits intoSciML:masterSciML/SciMLBase.jl:masterfrom

AayushSabharwal:as/save-idxs-fixAayushSabharwal/SciMLBase.jl:as/save-idxs-fixCopy head branch name to clipboard