code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          if self.alpha == 0.:
              return 1./2.*nu.log(R**2.+z**2./self.q2+self.core2)
          else:
              m2= self.core2+R**2.+z**2./self.q2
              return -m2**(-self.alpha/2.)/self.alpha
docs: NAME:
             _evaluate
          PURPOSE:
             evaluate the potential at R,z
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             Phi(R,z)
          HISTORY:
             2013-01-09 - Started - Bovy (IAS)