CRIT_ADD refactored, THRACIA_CRIT added

I approached this problem through the equations widget because I think it's the simplest solution

I edited combat_calcs because that's what the damage component calls, and also deals with very particular edge cases.

Thracia_Crit is setup to not be required in the code so people pulling this change won't have to add it in.

If you have issues let me know and I can change the implementation as you want.