Undetermined order of postprocessings collides with use of reference_tensor

[FynnBe]

As we define pre- and postprocessing steps per tensor they do not have a (clear) global order.
Some postprocessing steps reference other tensors. We for now just defined that these other tensors are always referenced before their own pre-/postprocessing. This assumption is arbitrary.

I propose to separate pre-/postprocessing from individual inputs/outputs to their own fields preprocessing/postprocessing and add tensor_id to all pre-/postprocessing kwargs.
This allows for user-control of the global processing order.

relevant postprocessings:

• scale_mean_variance
• scale_range

cc @bioimage-io/spec-dev