butterflies: updates and cleanups

src/modules/butterflies/butterflies_generations.c

src/modules/butterflies/butterflies_generations.h

src/modules/butterflies/butterflies_inline.h

src/modules/butterflies/butterflies_main.c

src/modules/butterflies/butterflies_main.h

src/modules/butterflies/butterflies_overlays.c

src/modules/butterflies/butterflies_overlays.h

src/modules/butterflies/butterflies_vital_rates.c