Implement Horndeski models in CCL

[philbull]

We can implement Horndeski modified gravity models by connecting CCL to the Hi_CLASS code (which is based on CLASS). This will allow CCL to make consistent predictions for observables in a broad class of scalar-tensor theories.

Part of this work will involve solving issue #12, linking to an external version of CLASS.

[elisachisari]

@philbull can we close this?

[c-d-leonard]

@philbull how are we feeling about this issue currently?

[damonge]

I think this will be too much work, and it will be indirectly solved anyway through Christos' work (since then we'll be able to use predictions from e.g. hi_class).

[damonge]

The calculator mode should solve this (you can just use hi_class to populate CCL). I'll close in a few days unless others chime in.

[mishakb]

I agree to that. This can be closed.