Consistent naming convention

We need to establish a consistent naming convention for our functions, classes and variable names.

We started to follow PEP8 and should move to this entirely.

This can also be checked with pylint, we need to stop ignoring invalid-name errors (only after fixing all of them though).

related to !513 (merged)

This involves rather big changes, also concerning the config files, we should do this in steps.

Here are some changes which require config adaptions, change

• nJets to n_jets
• njets to n_jets
• NN_structure to nn_structure
• nEpochs to n_epochs
• wp to working_point
• WP to working_point
• all the plot arguments need to be changed to the new API