New version of  Flows.ipynb
 Import png icons contributed by Yannick.
 Add optional argument `required_files`. Useful for restarting a flow from existent files.
 Add support for gw_kbpolicy in GW scripts
 Use separate module to store the abinit variables
 Fix issue due to the fermi level reported by Abinit (won't work in metals if NSCF run)
 Use fermi energy as reference energy in band structure plots
 updated docs
 Rename gw_kbpolicy to gw_qprange
Merge branch 'master' of https://github.com/gmatteo/abipy