code: def epifreq(self,R):
          
          return nu.sqrt(self.R2deriv(R,0.,use_physical=False)\
                             -3./R*self.Rforce(R,0.,use_physical=False))
docs: NAME:
          
             epifreq
          
          PURPOSE:
          
             calculate the epicycle frequency at R in this potential
          
          INPUT:
          
             R - Galactocentric radius (can be Quantity)
          
          OUTPUT:
          
             epicycle frequency
          
          HISTORY:
          
             2011-10-09 - Written - Bovy (IAS)