Added the norm_type argument to the grace.write user function.
This is in response to http://thread.gmane.org/gmane.science.nmr.relax.devel/7392/focus=7438.
This norm_type argument can either be 'first' or 'last' to allow different points of the plot to be
the normalisation factor. The default of 'first' preserves the old behaviour of first point
normalisation.
