Incorrect FLUKA initializer

The example scripts mars.cpp and water.cpp both have the old FLUKA interaction initializer.

These were missed because the CI doesn't build FLUKA.

i.e.

corsika::fluka::Interaction leIntModel{env};

instead of

auto const all_elements = corsika::get_all_elements_in_universe(env);
corsika::fluka::Interaction leIntModel{all_elements};