This is intended to avoid multiplying the # of biomes for all maps that would just like a wet/dry or a summer/spring cycle. Maps should be able to cherry-pick supported biomes from the seasonal biomes, and can then customize things in the map script depending on currentBiome().
TODO:
- would be nice to allow changing the biome description on a per-map basis / map description on a per-biome basis.
- Implement some sane defaults for e.g. lighting for each seasons.
- Use this for some maps (e.g. pyrennean sierra? Persians Highlands / fields of Meroe / Saharan watering holes etc.)
(I am not targeting A25 for this change)