Interactively update preprocessing parameters

[r-b-g-b]

It would be great to be able to adjust preprocessing parameters in real-time. #20 takes a big step towards this by registering the pipeline to the redis database, from which the web interfaces can read (and write). It also adds a new endpoint to the web interface at /pipeline. This endpoint displays the currently loaded preprocessing pipeline. To implement this we need two steps (that I can think of):

• /pipeline needs to be made reactive, currently it only displays the values
• we have to make the PreprocessingStep subclasses check the redis database with each new volume