Obstacle: Slowing 'swamps'

[AJFaraday]

From issue #2

Now there are some obstacles that are completely impassable 'rocks', use the same pattern to create some swamps.

• Swamps are unchangeable, they remain swamps throughout the game.
• They have a different move interval to plain space
  • This should be configurable, probably 6, as default.
  • Troops move off swamps in the same way as open space, but only on turns which are a multiple of the swamp move interval (see GridGame.spawning_turn for a pattern).