message: Update units to tech tree
  Howitzer also requires Ballistics
  Marine gets bonus against Howitzer instead of Artillery
diff: <iUnitClassMod>50</iUnitClassMod>
  </UnitClassAttackMod>
  <UnitClassAttackMod>
  - <UnitClassType>UNITCLASS_ARTILLERY</UnitClassType>
  + <UnitClassType>UNITCLASS_HOWITZER</UnitClassType>
  <iUnitClassMod>50</iUnitClassMod>
  </UnitClassAttackMod>
  </UnitClassAttackMods>
  </UnitAI>
  </UnitAIs>
  <PrereqTech>TECH_ASSEMBLY_LINE</PrereqTech>
  + <TechTypes>
  + <PrereqTech>TECH_BALLISTICS</PrereqTech>
  + </TechTypes>
  <BonusType>BONUS_IRON</BonusType>
  <iCost>200</iCost>
  <iAdvancedStartCost>100</iAdvancedStartCost>
  