Dev

This MR adds the following:

• Adding a new GENERAL option called FoldSplitVariable which holds the name of the variable used to determine which fold an event goes into. Default is eventNumber. The choice is then performed by applying the modulo operator %.
• Some clean up in the ConversionHandler, i.e. moving from .__ prefixes to ._ prefixes.
• Adding additional information to an Errormessage thrown by the OptionHandler which would be thrown if an unknown settings for a valid option was specified. The Errormessage now also contains all possible settings for the option. Also cleaning up this part of the code a bit.
• Improving the NegWeightSummary plot to also include the percentage of total yields to improve the quality of an argument that can be made based on it, i.e. to better evaluate the impact of negative weights pre-training.
• In DNNModel changes are applied to always overwrite the Verbosity setting to 1 in case config files are produced using the Optimise script to reduce the size of the output files when HTCondor is run.