Biome based structure spawning

Add the KCSG.BiomeStructGenExtension mod extension to your BiomeDef. You can have multiple of those on your def. Example of use below:

<modExtensions>
  <li Class="KCSG.BiomeStructGenExtension">
    <!-- A list of layout def and their commonality -->
    <structures>
      <li>
        <layout>Layout1</layout>
        <commonality>0.2</commonality>
      </li>
      <li>
        <layout>Layout2</layout>
        <commonality>0.6</commonality>
      </li>
      <li>
        <layout>Layout3</layout>
        <commonality>1.0</commonality>
      </li>
    </structures>
    <!-- False: will not check if cell are walkable -->
    <canSpawnInMontains>true</canSpawnInMontains>
    <!-- canSpawnInMontains need to be false -->
    <clearCellRadiusAround>2</clearCellRadiusAround>
    <!-- Count to spawn -->    
    <spawnCount>80</spawnCount>
    <!-- Should tile hiliness multiply count -->
    <countScaleHiliness>true</countScaleHiliness>
    <!-- Multipliers -->    
    <scalingOptions>
      <flat>0.5</flat>
      <smallHills>1</smallHills>
      <largeHills>2</largeHills>
      <mountainous>3</mountainous>
    </scalingOptions>
    <!-- Regnerate ore after spawn, 
         useful if you plan on spawning rocks
         and want ores to spawn randomly in it -->
    <postGenerateOre>true</postGenerateOre>
    <!-- Spawn on water... -->
    <canSpawnOnBridgeableTerrain>false</canSpawnOnBridgeableTerrain>
  </li>
</modExtensions>