Feature: add input warpscaling

[fabioseel]

Adds the possibility to 'warp' the input and by that get a 'foveated' vision.
It does so by sampling at full resolution in the center and then skipping more and more rows/lines to the outside, depending on

• warp_exp
  and the output size after warping defined through
• warp_h
• warp_w

Open TODOs:

• better hydra like setting of the parameters (currently can be set only directly via the samplefactory parameter) - perhaps in general a good idea to discuss how to deal with that
• documentation

[fabioseel]

@alex404 I created this pull request also because it shows an alternative way to implement the input transforms, maybe worth discussing whether this would be better (as opposed to the current way of 'hijacking' the input normalization)

Otherwise I think it's a nice feature for future experiments but not urgent atm, so for now it will 'just exist'