Feature request: `all_systems()`

[luke-kiernan]

It'd be very convenient to have an all_systems(sys_cat::SystemCatalog = SystemCatalog()) function, that returns a Dict{Tuple{SystemCategory, String}, PSY.System} of all the systems. I frequently find myself asking questions like "what's the biggest non-PSSEParsingTestSystems system with a generic 2 terminal HVDC line?" I've been writing one-off scripts for such things, along the lines of

for category in list_categories(SystemCatalog())
    for system in list_systems(SystemCatalog(), category)
        sys = build_system(category, system)
        # do stuff
    end
end

but this seems like a common enough use case that we should add it as a feature.