`Gnuplotpp` static plotting functions should return a `unique_ptr`

[LucaCiucci]

So that it is easier to manage and is can easily be converted to shared_ptr,
For example:

auto p = Gnuplotpp::plot(/* ... */); // p is 'unique_ptr'
std::shared_ptr<Gnuplot::Plot2d> p2 = Gnuplotpp::plot(/* ... */); // also valid