Parametrize GenericModel with objective type

[blegat]

We could have GenericModel parametrized by the objective type so that a solver can set it to MOI.ScalarAffineFunction{Float64} in its OptimizerCache type so that it won't have to worry about other cases.
For instance,
https://github.com/jump-dev/Clp.jl/pull/114/files#diff-18c6af231f3facc5ba10dd45231da349efda22ecd8d68a4661dee8027bc413a7R296-R297
is incorrect as it will throw a convert error if the objective is quadratic instead of an UnsupportedAttribute error.

[odow]

I also wondered about just storing three fields for the three objective types, or having a separate Objective structure.

[odow]

I also just noticed that the current approach leads to inference errors, because of this:

MathOptInterface.jl/src/Utilities/model.jl

Lines 694 to 695 in 830ef4e

	isempty(model.objective.terms) &&
	iszero(model.objective.constant) &&

I'll have a go after I finish #1468