Add to_dict utility method

[tilmantroester]

This adds a Cosmology.to_dict method and addresses #1159, as well as #1150. It allows round-tripping, even when the arguments are complex types:

    cosmo = ccl.CosmologyVanillaLCDM(
        transfer_function=ccl.emulators.EmulatorPk(),
        matter_power_spectrum=ccl.emulators.CosmicemuMTIIPk(),
        baryonic_effects=ccl.baryons.BaryonsvanDaalen19(),
        mg_parametrization=ccl.modified_gravity.MuSigmaMG()
    )

    assert cosmo == ccl.Cosmology(**cosmo.to_dict())

Currently this dumps everything into the yaml file when calling write_yaml, which can be quite awkward for all the internal data of the emulators. The upside is that the write/read yaml works for round trips but I think throwing an error until we have a cleaner solution for yaml serialisation for complex types might be cleaner.

Now raises an error when trying to use write_yaml with complex types.

[coveralls]

Pull Request Test Coverage Report for Build 8048774551

Details

• 0 of 32 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 97.484%

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Totals
Change from base Build 7986188385:	0.02%
Covered Lines:	6547
Relevant Lines:	6716

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💛 - Coveralls

[damonge]

LGTM