[Feature Req.]Elaborate more on k_bins and bin_ave in the tutorial

[jordanflitter]

In calculate_ps_coeval (and also in calculate_ps_lc), there is an argument called k_bins which can be used to specify either the centers of the k-bins or their edges (depending on the value of bin_ave). I wasn't able to find any reference to them in the tutorial. I think it could be helpful for new users of tuesday if these arguments were mentioned in the tutorial.