add spelleffect_data_t::average( special_effect_t )

[gastank]

when passed a special_effect_t argument, average() will determine the appropriate scaling based on the spell's attributes.

1. if the spell does not have SX_SCALE_ILEVEL, it will use player level scaling.
2. if the spell has SX_SCALE_ILEVEL and a valid item, it will use item level scaling with the item level capped by the maximum scaling level of the spell.
3. if the spell has SX_SCALE_ILEVEL, no item, a valid a maximum scaling level, it will use item level scaling with the item level equal to the maximum scaling level of the spell.
4. if the spell has SX_SCALE_ILEVEL, no vaild item, and no maximum scaling level, it will assert as this situation requires manual intervention.