Fog of war - consider tile elevation and obstacles

[QuintillusCFC]

Certain type of terrain affect fog of war.

Hills and mountains and volcanoes let you see farther, but also prevent you from seeing past them.

This issue is to consider and implement that.

We should probably do this by adding a Height property to TerrainType, and setting it appropriately when importing from Civ3.

TBD whether Babylon or Carthage.