Add MaxSpread to Warhead to allow setting a specific area of effect.
Ion Cannon now scorches the whole hit area, and uses higher Spread + low
MaxSpread to increase damage within the hit area, but keep area of effect
itself relatively low.
Additionally increased damage to make it less inferior to Nukes.
Merge pull request #5941 from reaperrr/spread-upgrade

Added MaxSpread for more control over maximum area of damage