code: def flattening(self,R,z):
          
          return nu.sqrt(nu.fabs(z/R*self.Rforce(R,z,use_physical=False)\
                                     /self.zforce(R,z,use_physical=False)))
docs: NAME:
          
             flattening
          
          PURPOSE:
          
             calculate the potential flattening, defined as sqrt(fabs(z/R F_R/F_z))
          
          INPUT:
          
             R - Galactocentric radius (can be Quantity)
  
             z - height (can be Quantity)
          
          OUTPUT:
          
             flattening
          
          HISTORY:
          
             2012-09-13 - Written - Bovy (IAS)