Updated the grace2images.py scripts created by the relax_disp.plot_disp_curves user function.
This was discussed in bug #20916 (https://gna.org/bugs/?20916) and the change suggested in the post
http://thread.gmane.org/gmane.science.nmr.relax.devel/3953/focus=4000.
Improved that both small and big letters for image types can be used on the command line.
