STY: Change ITK_OVERRIDE for override in whole code (find Modules -type f -exec sed -i 's/ITK_OVERRIDE/override/g' {} +)

(cherry picked from commit a8db8b4c)

This reverts commits:
3d4ab1ff
2dc562e6

Terminate MPI before the end of the main instead of the destructor.
Also clear FFTW threads at the end of DoExecute() not in the destructor.