Weighting factor for positive examples (conf['data']['positive_example_penalty']) only works for 'maxhinge' target #53

The only time in which positive_example_penalty appears in the codebase is in:

Need to extend it to the other target functions, remove it as a parameter, or document this more thoroughly.

Not as important for DIII-D datasets as it is for our JET datasets (for which the non-/disruptive classes are more imbalanced).

• Also, conf['training']['ranking_difficulty_fac']: 1.0 # how much to upweight incorrectly classified shots during training appears to perform a very related role, but instead within loader.py, mpi_runner.py, and performance.py.