code: def rho2theta(self, rho0, gamma):
          
          fac = np.sqrt(np.pi) * special.gamma(1. / 2 * (-1 + gamma)) / special.gamma(gamma / 2.) * 2 / (3 - gamma) * rho0
  
          #fac = theta_E**(gamma - 1)
          theta_E = fac**(1. / (gamma - 1))
          return theta_E
docs: converts 3d density into 2d projected density parameter
          :param rho0:
          :param gamma:
          :return: