Normalized coefficient of "decision" variables

[rtwalker]

I'm running into errors using the normalized_coefficient function on variables that are of type DecisionRef or KnownRef. If the decision variable isn't in the constraint, I'd prefer it to return 0 rather than throwing an error.

Of course I can do some filtering on the front end to avoid calling normalized_coefficient for constraints that aren't AffineDecisonFunctions, but for the sake of convenience/completeness, I wanted to check whether you would be open to a PR adding the following methods?

function JuMP._affine_coefficient(f::_VariableAffExpr{C}, decision::DecisionRef) where C
    return zero(C)
end

function JuMP._affine_coefficient(f::_VariableAffExpr{C}, known::KnownRef) where C
    return zero(C)
end

[martinbiel]

Fine by me! Looking at the JuMP definitions this should probably be the intended behavior anyway.

[rtwalker]

Great! I'll turn it into a PR a little later today