Native support of effect stages in skills and upgrades

I implemented the native support of effect stages in both skills and upgrades. Effect stages are e.g. ``Damage: 11 > 14 > 25``, ``Slowing Power: 7.5% > 15% > 22.5% > 30%`` or ``Amplify damage: 20% > 25% > 30% > 35%``.
By native support, I mean that skills and/or effects can have multiple stages and they get applied to each other correctly. The four possible combinations work natively for any effect:

| base effect | upgrade | result |
| :------------: | :----------: | :-----: |
| ``Damage: 10`` | ``Damage: +1`` | ``Damage: 11`` |
| ``Damage: 10`` | ``Damage: +1 > +2 > +3 > +4`` | ``Damage: 11 > 12 > 13 > 14`` |
| ``Damage: 11 > 12 > 13 > 14`` | ``Damage: +1`` | ``Damage: 12 > 13 > 14 > 15`` |
| ``Damage: 11 > 12 > 13 > 14`` | ``Damage: +1 > +2 > +3 > +4`` | ``Damage: 12 > 14 > 16 > 18`` |

Additionally, if the ``damage`` effect has multiple stages, the Nautsbuilder automatically calculates ``avg. damage`` if not already set.
Because of these changes, specific code for both Skølldir's Bash and Ayla's Evil Eye has been removed. Obviously, the spreadsheet has to be updated accordinly.