save CDM+baryon linear power spectrum

[SebastianBocquet]

So far, the class interface allows to store the total (linear) matter power spectrum. However, there are applications like the halo mass function where the power spectrum of CDM+baryons is needed. The camb interface already allows to store the cdm+baryons linear power spectrum, this PR allows to do the same for class (using the same variable name cdm_baryon_power_lin). Thanks also to @asmaamaz

[joezuntz]

This looks great, thanks so much! One request before I merge - could you make this optional, with it switched off by default, since it seems to take a bit of extra time to compute that not everyone will need. e.g. you could add a boolean configuration parameter save_cdm_baryon_power or something like that?

[SebastianBocquet]

Good idea. I've added save_matter_power_lin (defaults to True) and save_cdm_baryon_power_lin (defaults to False). This way, the old setup is run by default.

[joezuntz]

Thanks, this is great!