Silent failure for P(k) rescaling in cosmologies scale-dependent MG

[c-d-leonard]

CCL will allow you to define a cosmology with a scale-dependent mu / Sigma parameterization, and ask for a 'boltzmann_camb' or 'boltzmann_class' transfer function (rather than an 'isitgr') transfer function. If you do this, CCL will re-scale the power spectrum for you using a growth function computed from solving the quasistatic growth equation with mu. But, it will do something where it seems to basically ignore the scale-dependence, silently (see the k=0 case in ccl_mu_MG in ccl_muSigma.c - this is how it is called in this scenario from within growth_ode_system_muSig in ccl_background.c.

It doesn't really make physical sense to do this rescaling option when you have a scale-dependent mu anyway - you need to solve the full Boltzmann equations. We should throw an error if somebody defines a cosmology with these parameters and transfer function combination then tries to get the matter power spectrum.