`PGAS` requires an empty state vector to start with

[FredericWantiez]

There is an issue with the way we handle inner state container in PG and PGAS. For a model like this:

mutable struct Model 
   States::Vector{Float64}
end

PGAS would require the constructor of Model to create an empty array and the sampler would push new states / values into it.
For PG, that's the opposite. We need to start with a pre-allocated array (like zeros(T, N)).

[FredericWantiez]

#80 (comment)