Add a way to check if gnuplot is installed without showing figures

Could you add a way to check that gnuplot is installed? Currently, the only way I can see is to create a `Figure`, then try to show it or write it to a file. It would be nice if I could get my program to fail fast if gnuplot is not installed, before it gathers and processes a lot of data.

Maybe a possibility is to have `Figure::get_gnuplot_version()` return a `Result<GnuplotVersion>`.