Reward depends on NTG and TTC diff. Interpolation of sums of two functions.

* Reward function punishes large differences of NTG and TTC. This makes
  accel actions worthwile if they improve the NTG and TTC.
* Interpolation module allows to compute roots of sums of two linear or
  reciprocal functions.
* With that we could optimize NTG and TTC diff of the next two
  observations, but for some reason this does not work well in practice.
  Why?!