Add struct definitions for defining a world map.
Add struct definitions for defining a world map.

Summary:
* A Base struct that can be embedded into the rest of the structs
  defining common fields.
* Square defining a single square on the world map.
* Terrain defining the types of terrain that a square can have.
* Resource defining the type of resource that a square can have.
* Municipality

Depends on D70

Reviewers: swsnider

Reviewed By: swsnider

Maniphest Tasks: T29

Differential Revision: http://d.sil.as/D71

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