Testing P(k) with different neutrino mass, comparing with the CAMB code

[DriftingPig]

I tested the code that computes Pk from CAMB and CCL, with a focus on how the neutrino mass changes the Pk. Here are a few conclusions:

1. sigma8 is defined differently in CAMB and in CCL. In CAMB, for a fixed cosmology, sigma8 changes with redshift. For CCL, sigma8 does not change given a fixed cosmology. Thus I can't compare results from CAMB & CCL by specifying sigma8.
   For neutrino study, we need to fix A_s and Omega_m so that the low k region (k<0.01) does not change. The ratio of two Pk is 1 at low k region. I fix Omega_m by computing the Omega_nu first, then I subtract the value of Omega_nu from Omega_CDM.

2. When using A_s instead of sigma8, the pk from CCL and CAMB are very similar. However, they don't match exactly, as shown in the second plot.