Since the PETSc libraries incorporate timing of phases and operations,
we do not every recommend using PetscTimeSubtract()
The options database command -log_view activates
PETSc library timing. See Users-Manual: ch_profiling for more details, also
see PetscLogStageRegister(), PetscLogEventRegister(), PetscLogEventBegin(), PetscLogEventEnd() for how to register
stages and events in application codes.