Editing Spell research

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* It's now possible to carry out research in the Mage Guild screen, replacing spells in the slots.
* The cost is (2*spell level) of mercury, sulfur, crystals, gems, and 1000 gold. Research is limited to one per day, per town (doesn't prevent construction).
* The research removes the old spell from the slot and offers a choice of two random new spells (one new spell at level 5, no choice). It's possible to do further research in the same spell slot, but the heroes won't be able to learn the spell until the research is finalized. New spells will be generated according to the base certainty value by town type. Old spells (that ones that were in town or were not chosen after research) won't be generated again before every potential new one is cycled through.
* You may finalize the research at any point. From that point your heroes will be able to learn the spell you selected, but the spell slot will become permanent.
* A town's mandatory spells can't be replaced. Only one instance of research may be carried out in a town at any time. You'll have to finalize the research before working on a different spell slot.