Combat scripts: process damage modifications from elemental interactions -- fire on fire vs. ice etc.

[phylll]

Some kinds of elemental damage behave radically different against different types of target: fire damage is reduced by half when suffered by creatures of fire such as fire demons, and doubled when suffered by creatures of ice, and vice versa.

Characters' own elemental nature (not just affinity!) is registered in a new custom attribute element set by the GM.

How/where to store the attack damage type if not manually per known weapon such as Yorric's Firemole?

The resulting damage modifications are:

damage type	target.element	damage modifier
fire	fire	/ 2
fire	ice	* 2
ice	fire	* 2
ice	ice	/ 2

[phylll]

Requires #64