Added the scaling argument to the value.display and value.write user functions.
The idea comes from a suggestion by Angelo Figueiredo <am dott figueiredo att fct dott unl dott pt>
and was discussed at http://thread.gmane.org/gmane.science.nmr.relax.user/1428/focus=1430.
This allows the user to scale parameters to any value, for example scaling the Rex value to the
field strength dependent value.
