Added genPOI workaround for MystCraft

Possibly fixes issue #993.

Uses the number at the end of the dimension name as dimension number. This is most likely going to work with the way Forge keeps track of which numbers are taken; the Mystcraft FAQ says it stores a list of populated dimension numbers. Thus, I assume the number after DIM_MYST would be that dimension number.

In theory, this should work.

*In practice, I didn't test it.*