/// access K(i, j). Works only for dofs on this processor!!!!
Real PETScMatrix::operator()(UInt i, UInt j) const{
AKANTU_DEBUG_IN();
AKANTU_DEBUG_ASSERT(this->dof_synchronizer->isLocalOrMasterDOF(i) && this->dof_synchronizer->isLocalOrMasterDOF(j), "Operator works only for dofs on this processor");