Begun work on generating a 'ring' of cells
- Map.ring_around() method to obtain a ring
- test case to test the size of generated rings
Fixed bug where first cell of a ring was returned twice
Improved ring generation algorithm
Finished implementation of rings
- fixed wrong cell counts in tests
- added tests for radius 3 rings
Rewrote Map.neighbours() to catch missing neighbours
- neighbours() returns the union of all rings of the given radius and smaller
- default radius is 1
- fixes #1