Moving check for working point in train config configuration

The following discussion from !621 (merged) should be addressed:

• @mguth started a discussion: (+1 comment)

  this syntax can be shortened via

   working_point = float(val_params.get("working_point", eval_params["working_point"]))
  

  though it might be better to port this completely into the configuration part which would avoid a lot of code duplication.

  Can you open an issue for that?