Improve expect implementation and tests

[emstoudenmire]

Improve implementation and tests of expect to ensure return value is a dictionary when no vertices passed and a vector when a vector of vertices is passed.

Call -> Returns

expect(ψ, "Sz")   -> OrderedDictionary{Tuple{Int,Int}, Float64} 
 Note: sz[(1,2)] works

expect(ψ, "Sz", [v3, v1, v2]) -> Vector{Float64} 
Note: in that exact order    

expect(ψ, "Sz", (v1, v2)) -> Tuple{Float64, Float64} (falls out for free) 

expect(ψ, Op("Sz", v))  -> Float64    

Two notes on the whole-network case:

The type is OrderedDictionary, not Dictionaries.Dictionary. vertices(ψ) is a
NamedGraphs.OrderedDictionaries.OrderedIndices, so map over it yields the matching OrderedDictionary — an
AbstractDictionary keyed by vertex, iterating in vertices(ψ) order.

Element types stay concrete — Float64, not Any. Going through Indices/Dictionary directly widens to Any because
Dictionaries.jl sizes the output by inference and expect's contract(...)[] doesn't infer; OrderedDictionary's map
doesn't have that problem.

[mtfishman]

Looks good, thanks!