Concatenate transformations

What and Why

Sometimes, there may not be a simple step from one representation to another one, but by concatenating transformations, you can get form one step to another. As a (contrived?) example, to get from the unweighted DVR to the FBR, you could go via the weighted DVR.

This could be simplified by allowing chaining of transformations. At the end of the day, this only requires implementing a multiplication operator and a chaining transformation.

The drawback is that I am not aware right now of a good use-case that would require such a chaining, hence this issue has rather low priority.

Acceptance criteria

• It is possible to chain transformations with the product operator. The output behaves like the concatenation of the input transformations.
  • elementary errors should be caught (mainly destination of one trafo is not the source of another trafo), though not all bad things can be caught, because we keep only track of grids, not of the actual representation.
• consider adding a FbrOperatorND that utilizes such a product transformation; might be part of a followup issue.
• consider optimizing certain cases (e.g., N-dimensional FFT instead of doing N FFTs); might be part of a followup issue.