code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          r2= R**2.+z**2.
          r= nu.sqrt(r2)
          return -(1./r-self._a/r2*nu.arctan(r/self._a))/self._a*R/r
docs: NAME:
             _Rforce
          PURPOSE:
             evaluate the radial force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the radial force
          HISTORY:
             2015-12-04 - Started - Bovy (UofT)