Fallbacks for vector attributes

[odow]

As proposed by @tkoolen in #209: MOI.get with vector variable indices: is that really needed? I saw an overload in SCS that just broadcasted to the scalar function, https://github.com/JuliaOpt/MathOptInterfaceSCS.jl/blob/a0f8467245d8b0a1814cf7248b174c1462990ae5/src/attributes.jl#L51. Can’t that be the default fallback?

I forgot whether we had a discussion about default fallbacks...

[mlubin]

I don't remember a good reason to not implement the default fallback. Maybe ambiguity issues?

[blegat]

I am in favor of a fallback. If we use types that are not too specific like in

MOI.get(::AbstractInstance, ::AbstractVariableAttribute, ::Vector{VariableIndex})

it just forces solvers to do

MOI.get(::SolverType, ::SomeVarAttribute, ::Any) # Bad
MOI.get(::SolverType, ::SomeVarAttribute, ::VariableIndex) # Good