butterflies: array access unification

src/modules/butterflies/CMakeLists.txt

src/modules/butterflies/butterflies_dispersal.c

src/modules/butterflies/butterflies_generations.c

src/modules/butterflies/butterflies_initial_population.c

src/modules/butterflies/butterflies_inline.h

src/modules/butterflies/butterflies_overlays.c

src/modules/butterflies/butterflies_populations.c

src/modules/butterflies/inline_butterflies.h