Fixing bugs in NN_tools and preprocessing.py

This MR adds the following:

For the NN_tools.py:

• Fixes the bug when loading dips in the plotting_epoch_performance.py. You need to define the CustomObjectScope to load the model correctly.
• Adds a small work-around if jets are only loaded from one file. If you request 300k jets, this is the number which is loaded before the cutting. To ensure you load enough jets before cutting, increase the number of requested jets by 15%.
• Cleaning up some function definitions and adding doc-strings, comments etc.
• Standardize some function input variable names.
• Adding the variable_cuts from the training config also for the validation (was only added for evaluation up till now).
• Move the loading of the files outside of the loop of the calculate_metrics function (Loading is very time consuming).

For the preprocessing.py:

• Adding some comments.
• Adding a little work around that all samples in the config are prepared if --sample is not given when calling the preparation step.
• Adding a small fix to docs.

For the unit tests:

• Updated unit test control plots.
• Fixing some naming issues.