Can now take the dual of a grid

Taking the dual of a grid creates a new grid with one vertex
for each face of the original and one face for each vertex. This
allows the easy introduction of a new grid type, the dual of the
octagonal grid, which is a square grid in which each square is split
into four triangles.

Of the grid types currently present, square is its own dual,
honeycomb is the dual of triangles, cairo is the dual of snub-squares,
kites is the dual of great-hexagonal, and the dodecagonal ones would
have vertices with twelve edges, probably not practical. The others
all have duals that would introduce new classes of puzzle.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9395 cda61777-01e9-0310-a592-d414129be87e