Suggestion: Parameter the size of the world on size of phantoms/scanner

[robbietuk]

A voxelised phantom (activity or mumap) larger than the world will cause a crash.

We may however have a phantom that is an entire XCAT torso/body with different z lengths. For any particle, tracking stops when it escapes from the world volume. Therefore, we do not want to make the world too large by default because of computation time concerns.

We could therefore take the maximum length of the phantoms and scanner in each dimention and use that for the definition of world size. This would be an extention of #6 (4.)