Remove `prototile` from `TileUnit` constructor workflow

Prototiles don't really serve any functional role other than in setting up the tiling vectors.

Usually it is much easier to know the tiling vectors of a tiling. So it would be better either to remove prototiles entirely or if retaining them to come up with a satisfactory way of deriving them from the vectors. A method that works has been developed tiling_utils.get_prototiles_from_vectors() but doesn't always find 'nice' solutions.

Accompanying this change it would need to be possible to change the tiling vectors under the various transformations (scale, rotate, skew) applicable to TileUnits but this should not be hard.

Also check the use prototiles option in the tiled map making procedure.