Start of biome mode BeforeGroups to simulate the old behaviour

The biome mode already looks promising; maps look closer to the
original. Some more changes are needed. Also, it is not yet documented,
nor is it turned on automatically for old maps.
Remove some dead code

If it's actually useful it can easily be readded.
Don't allow duplicate groups

Previously, the second would overwrite the first without warning,
leading to strange behaviour.
Correctly calculate total group rarity

Calculation from earlier versions are now restored for the compat biome
mode. Still doesn't look for 100% for what it should look like.
Remove unused static variable

Even if the variable was used it would still need to be removed, as
static variables are very multiworld-unfriendly.
Document the new compat biome mode
Automatically put BiomeMode to BeforeGroups on old maps

If the setting NormalBiomes is present we have an old map, the BiomeMode
needs to be switched to avoid any surprises.

(Well, even this compat biome mode doesn't yet simulate the old
behaviour 100%, but this will be fixed before release.)
Fix duplicate settings reading

Merge conflict?