Use parametric type for model

[ocots]

          @ocots @jbcaillau Before that I forgot, you are using quite often a `foo::Union{Nothing, ...}` inside your structures for the type of the attributes. The compiler doesn't like that and it leads to additional allocations because it needs to check at runtime that it's a Nothing or not.

You can change that for foo::T and at the top of your structure T <: Union{Nothing, ...}.

Originally posted by @amontoison in #271 (comment)

[ocots]

We could use the builder pattern according to @gdupont1.

[gdalle]

Just popping by to say that I can advise on this too if needed, following our conversation at the Julia & Optimization Days