new default branch and development

[tjhei]

We previously had the default branch set to the branch corresponding to a specific version of deal.II like dealii-9.5, while the master branch was used for development and pull requests (with me cherry-picking all commits over to the currently active branch. Starting today this changed:

1. The default branch is now the master branch and this is where pull requests should be done to (this is now easier because one does not need to switch branches to master first)
2. The master branch now always installs the latest deal.II release (you can still install deal.II master by choosing master inside candi.cfg)
3. The branches dealii-9.x will no longer be updated.
4. I will still tag releases to have fixed points to go back to.