Add AbstractScalarFunction and AbstractVectorFunction

[blegat]

No description provided.

[mlubin]

Why is this extra level of abstraction needed?

[blegat]

I wouldn't say it is needed but it might be helpful to the solver wrappers. For example they might want to do:

addconstraint!(m::..., f::AbstractScalarFunction, s) = addconstraint!(m, MOI.vectorize(f), MOI.vectorize(s))

so that they do not need to implement the scalar versions (vectorize still needs to be defined). The issue is that it is not going to play well with NumberOfConstraint...

[mlubin]

That's not quite correct because the constraint references will be of the wrong type.

[mlubin]

I feel like this is better defined as Union{ScalarVariablewiseFuncton,ScalarAffineFunction,...} directly in the solver interface, rather than adding an extra level to the type hierarchy in MOI.

[mlubin]

I'm going to close this for now.