v0.5

Numerous changes, fixes and additions:

Full-flux-surface capability
New physical Fokker-Planck collision operator
Periodic boundary conditions for simulations with low magnetic shear
Added option to calculate drift terms implicitly (as an extra implicit step)
Better support for shared memory in the response matrix framework
Consistent handling of the k_perp = 0 mode for simulations with radial variation
Numerous performance enhancements, done by optimizing the use of CPU cache
Added the option to build with CMake, rather than make
Parallel nonlinearity now functions properly when radial profile variation is included
git commit number is now printed out in the stella splash screen