Describe the feature

Make possible to change worldtype (not environment !) in an existing world :
using command, or directly in the world.yml file.

How is the feature useful to all Multiverse users?

A bug using a world generator has change the 'world type generation' of my world to flat (from normal).
So, now, I'd like to set the world type to normal. But that not possible - to my knowledge - to do that with command (no worldtype property in the [/mv modify] command).
I propose to make that possible, and its can be useful for other users have same problem than me (and this problem is just vital to patch :/)
Thank in advance !

Agreements

• I have searched for and ensured there isn't already an open issue regarding this.

Just some insight to this, which might be food for thought for anyone trying to implement this.

World Type (and environment) is stored with the level (via dat files in the world folder), and Multiverse does keep track of this value in the world config, but it doesn't actually reflect what the level is loaded with. When we pass world info to Bukkit's createWorld (this is also used to load worlds -- yes, the naming is weird), the server basically ignores whatever we tell it about the type and environment and just loads it from the level data.

Ultimately, this means we may need to find a way to edit the level data itself (which can be problematic, and may or may not be easy to do). This is also a potentially dangerous operation to let people do, for obvious reasons.