Reparametrise NLA model in `c1_rhocrit`

[ntessore]

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A very good point that was raised by @ahallcosmo in a different context is that, in the NLA model, the parameter $C_1$ depends on cosmology, but the product $C_1 \rho_{\rm crit}$ is independent of cosmology. To make things easier for users, we should adopt c1_rhocrit as the input parameter of the model, with a default value of either 0.0134 (rounded) or 0.013876831360184281.