Access _gpaw only though gpaw.cgpaw
- add gpaw.cgpaw module which provides everything in _gpaw.
- rest of code accesses the C code only through gpaw.cgpaw.
- almost all changes are automatic (
sed -i s/..../..../
).
Related: #1138
(This MR doesn't close the issue since this just improves the level of control over initialization. I haven't checked exactly when the C extensions are in fact imported)