This pull request removes the (unused, deprecated) primordial_solver. This was left in the repository initially as a historical note, but it's continued inclusion may generate confusion: it is and has been deprecated for some time. Its place in repository history should be enough for historical purposes.
Additionally, vestiges of an experimental CVODE solver are removed in this pull request.
Everything looks good to me - a lot of files are changed, but it's all very local. I'm going to accept the PR momentarily.