Migrate speed, damage/armor and firepower modifiers to integer

Current state:
SpeedModifier seems to work fine.

DamageModifier currently does about twice as much damage as it should, probably a misplaced division op, haven't found the error yet.

FirepowerModifier leads to zero damage of tanks vs. infantry for example, so there's probably a misplaced division op as well.
Can't use .Sum(), unfortunately, since for things like Invulnerable or Ironcurtain that return 0 for GetFirePowerModifier the end result will be non-zero.