add initial sketch of lensing lite lhood

[timothydmorton]

@msyriac here's an initial sketch, but we need some realistic fake data and covariance matrix to test this better.

[cmbant]

I've been making some changes to Cobaya to support external likelihoods, where the external likekihoods inherit from Cobaya classes but can also be used directly outside Cobaya. I've made an example package for the Planck lensing likelihood https://github.com/CobayaSampler/planck_lensing_external - see the tests for examples of usage. Would something like this work OK for you for SO?

[timothydmorton]

Thanks @cmbant- I've been keeping an eye on this and that looks like a great model for us to follow with SO. I'll ping if I have any questions; hopefully will have a chance to look more into this this week. These updates require the external_modules cobaya branch, correct? Does following these instructions also require the external_modules branch, or should that work with the release version? Just asking because I tried briefly to re-write this lensing-lite demo as a class and had some issues.

[cmbant]

Yes, this requires the external_modules branch, the docs for which are here:
https://cobaya.readthedocs.io/en/external_modules/

(you could also use exactly the same code as the example Planck lensing likelihood, simply replacing the data files)

[timothydmorton]

That's great! I wanted to do a bare-bones thing to make sure I understood all pieces before trying to subclass _CMBLikes, but great to know that that would be easy. Will let you know if I have more questions!