Add CAMB

[nstarman]

Signed-off-by: nstarman [email protected]

Description

Adding the CAMB wrapper library.

PR Checklist

• Give a detailed description of the PR above.
• Document changes in the CHANGES.rst file. See existing changelog for examples.
• Add tests, if applicable, to ensure code coverage never decreases.
• Make sure the docs are up to date, if applicable, particularly the docstrings and RST files in docs folder.
• Ensure linear history by rebasing, when requested by the maintainer.

[nstarman]

This needs to wait until we have installable versions of everything else.