Rearrange include list in module definition so that "defintions.jl" is above "spatial_structure.jl"

[fieldofnodes]

From #11

When I forked the repo, re-arranged the include as

include("genetics.jl")

include("temperature.jl")
include("temperature_responses.jl")

include("life_stages.jl") 
include("organisms.jl")
include("definitions.jl")  

include("spatial_structure.jl")
...

I was able to call assign_migration!(network, migration_data, species); without error.

Though if I ran the package locally as a module by calling:

using Pkg
Pkg.activate(".")
include("src/GeneDrive.jl")
using .GeneDrive

It works as is in your package.