Make FormulaData.xml more useful

This is related to the party exp formula.

I've seen topics smth like "exp is gained too much in party" and the only one proposition to solve it - set party gap. But in any case party gap is not a good solution and exp part in FormulaData still be almost unuseful. 

My suggestion: move party exp formula to lua script or keep it in FormulaData, but move variables to the "normal variables". I mean allow to reuse the same variable in the same formula and have possibility to change variable place.

So I propose to use normal variable names, e.g. <Formula ID="4" Data="{soloParty}*{partyAddExp}*{memberLevel}/{totalLevel}" />

In this case a user will be able to change variable place/use the same variable multiple times/remove some variable in the middle and so on as it's needed.

I don't think it's a big issue to parse this variables in a code and use it, but very helpful feature for a users.