Interface with DifferentiableExpectations.jl

[BatyLeo]

Now that DifferentiableExpectations is registered, we can update InferOpt internals to rely on it.

[gdalle]

For Perturbed, define a new struct containing a Reinforce.
Identify additive and multiplicative settings based on what the struct contains?

struct Perturbed
    # other stuff
    re::Reinforce
end

do_stuff(::Perturbed{Reinforce{<:Normal}})  # ~ additive
do_stuff(::Perturbed{Reinforce{<:LogNormal}})  # ~ multiplicative