Better integration with Abinit (YAML format available in my Abinit developmental version)

Hi Shyue,

I'm gradually moving to pymatgen the new workflows for Phonon calculations with Abinit.
Many of these new features will require Abinit 7.6 that is presently under development.
Note that now abinitio uses the configuration directory $HOME/.abinit/abipy to store important
configuration parameters. The unit tests in abinitio will create this directory the first time we run the test suite.
Sorry for this side effect, but I really need these configuration files to simplify the interface with the end-user.