DWI::normalise_grad() should not modify b-value

Adjusting the b-value according to the norm of the gradient vector is
now handled by DWI::scale_bvalue_by_G_squared().
Fix read-write functionality in compressed image formats

GZ image handler wrapper was not having the header contents filled with data on image load. Therefore, if the image was opened read-write, the re-written header would contain uninitialized data.
Merge branch 'master' into C++11