module to find and validate the gnuplot executable

 Alien::Gnuplot verifies existence and sanity of the gnuplot external
 application. It only declares one access method, Alien::Gnuplot::load_gnuplot,
 which does the actual work and is called automatically at load time.
 Alien::Gnuplot doesn't have any actual plotting methods - making use of
 gnuplot, once it is found and verified, is up to you or your client module.