Refactor .objective field of Utilities.Model into a struct

[odow]

This simplifies the Utilities.Model quite a bit.

Currently needs test_broken due to the isapprox issue #1484

[blegat]

Could we do something like:

struct UnionScalarFunction{T}
    single_variable::Union{Nothing,MOI.SingleVariable}
    scalar_affine::Union{Nothing,MOI.ScalarAffineFunction{T}}
    scalar_quadratic::Union{Nothing,MOI.ScalarQuadraticFunction{T}}
end

and have

mutable struct ObjectiveFunctionContainer{T,F} <: MOI.ModelLike
    ...
    objective_function::Union{Nothing,F}
end

and MOI.Utilities.Model{T} would use ObjectiveFunctionContainer{T,UnionScalarFunction{T}} while Clp and may others would use ObjectiveFunctionContainer{T,MOI.ScalarAffineFunction{T}} and COSMO would use ObjectiveFunctionContainer{T,MOI.ScalarQuadraticFunction{T}} for their default cache (#1381).
Note that in recent Julia version, MOI.Utilities.Model{T} just prints as MOI.Utilities.Model{T} if I'm not mistaken so the printing argument for avoiding parametrized types does not hold anymore :)

[odow]

Coverage failure is because of poor testing of filter_variables from Utilities/functions.jl.

[blegat]

This is a nice improvement but it does not closes #1470. #1485 (comment) would close it but it can be done in a separate PR.