Suggest a example, which is easy to understand, of function 'hcubature'.

The integrand in the example now is written in the form of _inline_ function.

But, I believe it's easier to understand how to use 'hcubature' when the integrand is written in "_caller_" style. Because it's more often using multi-dimensional integrals of more complex function than f(x, y)=(x^3)*y.

I mean that maybe the example of 'hcubature' could use the following codes.
```julia
# ORIGINAL example
hcubature(x -> begin println(x[1],",",x[2]); x[1]^3*x[2]; end, [0,0],[1,1])
```

```julia
# SUGGESTION example
function f( x::Vector{Float64} ) # the integrand i.e., f(x, y)=x^3 y
println(x[1], ",", x[2])
return (x[1]^3) * x[2]
end
hcubature(f, [0,0],[1,1])
```