Expansion of the singular_values() function.
The basis_set argument has been added to allow the selection between {Sxx, Syy, Sxy, Sxz, Syz} and {Szz, Sxxyy, Sxy, Sxz, Syz}.
Authored by: bugman 2007-12-19
Parent: [r4215]
Child: [r4217]
