preparations for multi-scale simulations

See merge request !1

- renamed functions to match struct name
- better argument names
- added asserts

# Conflicts:
#	src/modules/butterflies/butterflies_dispersal.c
#	src/modules/butterflies/butterflies_generations.c
#	src/modules/butterflies/butterflies_populations.c

- new functions to access input grids (overlay, environment)
  these will be used to translate between input layer coordinates
  and population grid coordinates

- overdue helper function to set environment values

- helper function to check to copy and verify grid dimension info