Added the chi_surface=None to the backend function.
When None, it will try to find reasonale chi2 values. These will define surface levels for the Innermost, Inner, Middle and Outer Isosurface.
task #7792: (https://gna.org/task/?7792) Make the dx.map write suggest chi surface values.
Authored by: tlinnet 2014-05-09
Parent: [r23137]
Child: [r23139]
