Allow sigma8 as an input when using CAMB for nonlinear power spectra

[tilmantroester]

This PR rescales the primordial power spectrum in CAMB before computing the non-linear power spectrum, allowing using sigma8 as an input.

By default, INTEGRATION_SIGMAR_EPSREL is set to 1e-5, which results in a relative error of 1e-3 on sigma8. We might want to push this a bit.

[tilmantroester]

The failing CI is due to CLASS not supporting cython 3.0.0: lesgourg/class_public#531

[damonge]

So it's because of an update in gha?

[tilmantroester]

I think because class is not forcing its cython dependence to be less than 3.0.0, the class installation now fails because cython updated to 3.0.0, which breaks class.

[damonge]

OK, I say we wait a couple of days and see if class fix that. Otherwise we can try to force gha to use a previous cython version (disabling the class tests would be kind of tragic)

[hsinfan1996]

classy 2.8.0 might work. Versions>2.8.0 will install Cython 3.0.0 when being installed.

[hsinfan1996]

classy==2.8.0

[damonge]

No luck, it seems...

[hsinfan1996]

No luck, it seems...

It doesn't work here. pip (or mamba?) still installs the latest Cython I guess.

[hsinfan1996]

Does it mean only 2.9.4 and below are pip installable?
lesgourg/class_public#527 (comment)

[damonge]

@tilmantroester tests can now run, but some seem to be failing.

[tilmantroester]

Can someone explain this test:
https://github.com/LSSTDESC/CCL/blob/master/pyccl/tests/test_power_mu_sigma_sigma8norm.py#L48-L61 ?

From this my understanding is that if sigma8 is given, then setting mu and Sigma should not have any effect because any rescaling of the power spectrum is undone by the sigma8 normalisation?

[damonge]

Humm, I think it's just testing that initialising power spectra with A_s or its associated sigma8 gives the same result, even when you're passing MG parameters != 0

[tilmantroester]

Ah, my confusion was that the input sigma8 is including MG. Should be fixed now. Looking at the MG implementation, I'm a bit worried though. It's quite opaque and the C-layer does things like creating new C cosmology structs. Since we don't support the C API any more, is that safe?
Would an API like the baryons, which modifies the power spectra (and growth rates in this case) make sense, instead of having mu and Sigma as first-class cosmology parameters? Especially since that only applies to linear scales, which are of restricted use for the LSS stuff CCL is for.

[coveralls]

Pull Request Test Coverage Report for Build 5640753554

• 34 of 34 (100.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.004%) to 97.443%

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Files with Coverage Reduction	New Missed Lines	%
pyccl/baryons/baccoemu_baryons.py	1	98.63%

Totals
Change from base Build 5619969593:	0.004%
Covered Lines:	5679
Relevant Lines:	5828

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💛 - Coveralls

[damonge]

LGTM!