Add Cura III spell with its Afflatus Misery Bonus

Cura III is now learnable and has its correct base price, mp, cast times, etc set according to BGWiki

Implementation is very similar to the implementation of Cura/Cura II: largely semi-educated guesswork based off of the best available info on BGWiki, as I could not find a concrete cura formula anywhere. Should be reasonably close, workable.