code: def dvcircdR(self,R,phi=None):
          
          return 0.5*(-self.Rforce(R,0.,phi=phi,use_physical=False)\
                           +R*self.R2deriv(R,0.,phi=phi,use_physical=False))\
                           /self.vcirc(R,phi=phi,use_physical=False)
docs: NAME:
          
              dvcircdR
          
          PURPOSE:
          
              calculate the derivative of the circular velocity at R wrt R
              in this potential
  
          INPUT:
          
              R - Galactocentric radius (can be Quantity)
          
              phi= (None) azimuth to use for non-axisymmetric potentials
  
          OUTPUT:
          
              derivative of the circular rotation velocity wrt R
          
          HISTORY:
          
              2013-01-08 - Written - Bovy (IAS)
          
              2016-06-28 - Added phi= keyword for non-axisymmetric potential - Bovy (UofT)