Container for MG params

[tilmantroester]

This PR introduces a ModifiedGravity interface to the Cosmology class which encapsulates MG parametrisations (just mu-Sigma for now). Under the hood (i.e. C-layer) nothing has changed.
The motivation is to reduce clutter in the Cosmology interface and allow different MG parametrisations in the future without breaking the API.

Instead of

cosmo = ccl.Cosmology(
    Omega_c=0.25,
    Omega_b=0.05,
    h=0.7,
    A_s=2.1e-9,
    n_s=0.96,
    mu_0=0.1, sigma_0=0.2,
)

the new API is now

from pyccl.modified_gravity import MuSigmaMG
cosmo = ccl.Cosmology(
    Omega_c=0.25,
    Omega_b=0.05,
    h=0.7,
    A_s=2.1e-9,
    n_s=0.96,
    mg_parametrisation=MuSigmaMG(mu_0=0.1, sigma_0=0.2),
)

This doesn't use CCLNamedClass at the moment due to a conflict with dataclass (see #1118).

[coveralls]

Pull Request Test Coverage Report for Build 5743350713

• 37 of 37 (100.0%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 97.448%

---

Files with Coverage Reduction	New Missed Lines	%
pyccl/cosmology.py	1	99.26%

Totals
Change from base Build 5694401474:	0.01%
Covered Lines:	5805
Relevant Lines:	5957

---

💛 - Coveralls

[damonge]

@tilmantroester , I'm not sure it's worth trying to make this a CCLNamedClass. This is more appropriate for things like halo model ingredients, baryonic effects, or emulators, where there are a gazillion parametrisations and you want to sift through them quickly.

[tilmantroester]

Fair enough. Changing this wouldn't affect the API so this could be made a CCLNamedClass sometime later when there are multiple MG models.

[damonge]

Thanks @tilmantroester . Looks good to me, just a couple of minor things before merging.

[tilmantroester]

Are there opinions on the spelling of "parametrisation"? Do we use AE spelling and use z instead of s?

[damonge]

I think AE for the most part (although I find myself being very inconsistent)

[tilmantroester]

Switched to "parametrization" now.

[damonge]

LGTM!