CLASS growth factor

[timothydmorton]

It would be a good complement to #63 to also implement a CLASS growth factor, analogous to what is done for CAMB (https://github.com/steven-murray/hmf/blob/b1f8f068d42518786fb40da2e323de7f59a935ea/src/hmf/cosmology/growth_factor.py#L383), just to have full consistency. Pinging @jlashner.

[steven-murray]

That seems reasonable -- does the CLASS growth factor do anything different to eg. CAMB?