code: def _mass(self,R,z=0.,t=0.):
          
          if z is None: r= R
          else: r= numpy.sqrt(R**2.+z**2.)
          return (r/self.a)**(3.-self.alpha)/(3.-self.alpha)*special.hyp2f1(3.-self.alpha,-self.alpha+self.beta,4.-self.alpha,-r/self.a)
docs: NAME:
             _mass
          PURPOSE:
             evaluate the mass within R for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             t - time
          OUTPUT:
             the mass enclosed
          HISTORY:
             2014-04-01 - Written - Erkal (IoA)