unite registration steps into one transformation

[neuronflow]

The preprocessor transforms the images multiple times, leading to multiple resampling steps. Registration purists might find this provocative.

We should evaluate whether it is possible to do all transformations simultaneously and, therefore, have just a single resampling step. This is tricky to implement, given that we support multiple registration backends.

[neuronflow]

@sarthakpati this was the issue I mentioned.

[sarthakpati]

Understood. We can lump this in with the other stuff that needs to be done (especially #117), and I am happy to take a stab at it once we have scoped them out.