More flexible axis ordering in geometries

[kohr-h]

Our choice of axis ordering is reasonable when one wants to deal with projection images one at a time, but for other purposes it may be more reasonable to have a different ordering (for example to match the ordering of the backend, thereby removing overhead in copying and transposing data).

My suggestion for geometries in general is that we remove the lumped together attributes partition, params and grid, thus making geometries agnostic of axis ordering. The "consumers" of geometries (i.e. mostly RayTransform) should be responsible for handling axes in the data.

I think this change would only affect internal stuff and not break any user code.

[kohr-h]

Related to #152

[adler-j]

Frankly I agree. The current ordering is somewhat arbitrary to enforce on a global scale. It also causes issues for geometries that don't really have any motion.