Shifted from argparse to optparse in the grace2images.py scripts from relax_disp.plot_disp_curves.
This is associated with 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.
The argparse module is only available from for Python 2.7.3 (the version with many Python 3 features
backported) and Python >= 3.2. The module has been replaced with the similar optparse module as
used by relax, and which available in all Python version supported by relax.
