3D grids

[kaya3]

At the moment only 2D grids are supported, but it would be good to support 3D grids. This will necessarily require changes to many parts of the compiler:

• Need to decide if a program with multiple grids must have them all 2D or all 3D, and choose a syntax for specifying whether a program or grid is 2D or 3D. (Perhaps grid {dim=3} ...?)
• Tokenizer/parser needs to support that syntax and allow for 3D pattern literals. Probably use // for the separator in the z direction.
• Pattern types and the Pattern class need a depth property.
• 3D symmetry groups and functions to apply 3D symmetries to patterns.
• If 2D and 3D grids can coexist, need to decide the semantics of a map statement between grids of different dimensions (or perhaps have a separate project statement?)
• Position attribute expressions need to allow .z, but only for positions in 3D grids.
• Grid index expressions in the compiler need to include a z * width * height term. Ideally, the compiler should not emit declarations like atZ = ... for 2D grids, but this adds complexity.