Implements `SpawnsTiberiumType` for TerrainTypes.

[CCHyper]

Closes #624

This pull request implements a new key for TerrainTypes to allow overriding the Tiberium which Tiberium trees spawn.

[TerrainType] SpawnsTiberiumType=<TiberiumType> - The type of Tiberium this terrain object will spawn (only if it has IsSpawnsTiberium=yes). The value here should be the same as the name of the Tiberium types section (e.g. Riparius). Defaults to the first entry in the [Tiberiums] list.