Improve performance of the Square root filter

[guilhermebodin]

Some steps on the square root filter can be optimized with auxiliary functions, for example:

U2star[:, :, t] = Ustar[range1, range2]
sqrtF_kf[:, :, t]  = Ustar[range2, range2]

These steps could be replaced with

function fill_U2star!(U2star::Array{T, 3}, Ustar::Matrix{T}, prods::Int, t::Int) where T
    for i in 1:prods, j in (prods + 1):(prods + 2)
       U2star[i, j, t] = Ustar[i, j]
    end
    return
end

function fill_sqrtF_kf!(sqrtF_kf::Array{T, 3}, Ustar::Matrix{T}, prods::Int, t::Int) where T
    for i in (prods + 1):(prods + 2), j in (prods + 1):(prods + 2)
        sqrtF_kf[i, j, t] = Ustar[i, j]
    end
    return
end