Deal with in-place and out-of-place function in StochSystem

[oameye]

To deal with in-place and out-of-place functions in StochSystem we can do something similar as they do in the SciML Ecosystem for SDEProblem or ODEProblem. Something like:

# Define a parametric type
struct StochSystem{iip}
    f::Function
    pf::Parameters
    u::State
    σ::Float64
    g::Function
    pg::Parameters
    Σ::CovMatrix
    process::Any
end;

function StochSystem{iip}(...) where iip
	...
end

[reykboerner]

Have you already implemented this in one of your PRs @oameye?