New approximation for shear profiles

[m-aguena]

Implement the 2nd order approximations for shear profile:

$\Large g_t=\frac{1-\frac{\langle\beta_s^3\rangle}{\langle\beta_s^2\rangle}\kappa_\infty+\frac{\langle\beta_s^2\rangle}{\langle\beta_s\rangle}\kappa_\infty}{1-\frac{\langle\beta_s^3\rangle}{\langle\beta_s^2\rangle}\kappa_\infty}\langle\beta_s\rangle\gamma_\infty$,
from Seitz & Schneider et al 1997 (Eq. A2.5)

[hsinfan1996]

https://arxiv.org/pdf/astro-ph/9601079