Rework git branches
@loriab and @LecrisUT have convinced me that there needs to be a new major release to update the CMake system.
The issue is that the current master branch has broken the API in a nontrivial way from the stable releases. To avoid breaking the API multiple times, we would either have to push through the big API changes to include support for double hybrids and local hybrids - which is way too much work - or revert the API changes to allow releasing what is good in master to the world.
The plan is to rename the master branch as legacy, and to start a new release branch called stable. Once there's been a release, I will start a new branch called devel where development will happen. Whenever there is a new release, devel will get merged back into stable.
This way projects can always point to the latest release of Libxc, and release managing should become a bit easier.