Merge branch 'fix_inthist_sum' into 'master'

fixed InteractionHistogram::operator+()

See merge request AirShowerPhysics/corsika!203

Framework/StackInterface/comp

-TestParticleInterface2<TestParticleInterface<corsika::stack::StackIteratorInterface<corsika::stack::CombinedStackImpl<corsika::stack::CombinedStackImpl<TestStackData, TestStackData2>, TestStackData3>, CombinedTestInterfaceType2, corsika::stack::Stack<corsika::stack::CombinedStackImpl<corsika::stack::CombinedStackImpl<TestStackData, TestStackData2>, TestStackData3>, CombinedTestInterfaceType2> > > >::content
-