code: def _mass(self,R,z=0.,t=0.):
          
          if z is None: r= R
          else: r= nu.sqrt(R**2.+z**2.)
          return 2.*nu.pi*self.rc**(3.-self.alpha)*special.gammainc(1.5-self.alpha/2.,(r/self.rc)**2.)*special.gamma(1.5-self.alpha/2.)
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:
             2013-XX-XX - Written - Bovy (IAS)