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

So that it is easier to manage and is can easily be converted to `shared_ptr`,
For example:
```cpp
auto p = Gnuplotpp::plot(/* ... */); // p is 'unique_ptr'
std::shared_ptr<Gnuplot::Plot2d> p2 = Gnuplotpp::plot(/* ... */); // also valid
```