Clean up the ParamNet benchmark

[mfeurer]

The ParamNet benchmark apparently does not provide enough information. We need to:

• add information about the paper first using it (https://ml.informatik.uni-freiburg.de/papers/18-ICML-BOHB.pdf).
• add a link to the actual description of the surrogate benchmark (https://ml.informatik.uni-freiburg.de/papers/18-ICML-BOHB-supplementary.pdf).
• document that the cost to minimize is the misclassification error.
• limit the file to only a single class implementing THE ParamNet actually used by BOHB.