We don't follow the abstraction levels in `area_differentials` variable

[MemoOlv]

We have a variable area_differential that contains specific information from transects. This do not follow the abstraction levels. This information must be an input intead of being defined here.

We propose:

• Extract area_differentials as JSON input to CLI commands:
  • Apply in write-bird-densities
  • Apply in write-bird-transect-densities