Script: fix in _create_statement(): "undefine" and $name are now correctly separete as words.
Script: use @GNUPLOT_COMMAND instead of $GNUPLOT_PATH to run gnuplot. "--persist" command-line option is set by default to keep plot windows alive.
Add ProcessManager. Spawning a gnuplot process is refactored into that module. Use IPC::Open3 instead of IO::Pipe because open3 returns the child's PID.
ProcessManager: close the reader of gnuplot output because I don't use it.