Make this repository cpymad-only

This is the cpymad part of #30.

I think, it makes sense to use this repository for the new cpymad-only
repository since most of the issues are specific to cpymad. I suggest to then
renaming the repository to `cpymad` and adding a `pymad` repository at the
last release, with a comment in the README, referring to the new split
repositories.

I am unauthorized to do any administrative work like renaming / adding new
repositories in the pymad github organization, so you will have to do this.