document the finitediff code and add prefetching and unrolling for ILP to the Vc implementation
document the finitediff code and add prefetching and unrolling for ILP to the Vc implementation

VcDevel/Vc,chr-engwer/Vc,chr-engwer/Vc,chr-engwer/Vc,VcDevel/Vc,VcDevel/Vc,VcDevel/Vc