Add generic SquareTilemap class

[paulcscharf]

• [x] size of tile map should be arbitrary (as available memory and integer coordinates allow)
• [ ] needs support to translate between real positions and tiles, like
  • vector2 tile.Center (maybe any corner, plus single coordinate sides)
  • tile = tilemap.TileAt(vector2)
• [ ] should allow for arbitrary position offset
• [ ] size of square should be arbitrary (any float/double)
• [x] allow for easy enumeration of tiles
• [ ] allow to enumerate tiles intersecting rectangle/ray

This type could be used as regular tile map, or to divide game world space as acceleration data structure. With the ray-enumeration it can further be used to assist ray casting.

To consider:

• use plain vectors or SpaceTime2 types as coordinates?
• use int vector as tile positions (#42)?
• include tile-offset as well? (allows for easily centering tile map around 0,0 tile for example)