Add spectrum and RG running plot scrips for the user.
In the MSSM they can be used like this:

  $ gnuplot models/MSSM/MSSM_plot_rge_running.gnuplot -persist
  $ gnuplot models/MSSM/MSSM_plot_spectrum.gnuplot -persist

Note: Of course, the user has to generate the data files
"@ModelName@_rge_running.dat" and "@ModelName@_spectrum.dat" first.
This can be done by changing `#if 0' to `#if 1' in the spectrum
generator `models/<model>/run_<model>.cpp', recompiling and running
the spectrum generator.