Regression support and additional improvements of the code

This MR implements regression support and closes #11 (closed).

The new model Type introduced is Regression-DNN. The regression target and its label are provided via RegressionTarget and RegressionTargetLabel.

In addition the following items are changed:

• Conversion script was re-written to make future changes easier.
• Introducing general option RunONNXConversion (default = False) to turn on/off ONNX conversion
• Allowing minDelta values of >1 now
• Adding huber_loss as possible loss function
• Allow for output scaling of y for regression
• Splitting up plot handlers into dedicated classes
• Some label improvements for regression plots
• Allowing for the Verbosity to be defined
• Simplyfing code structure such that it should work without singularity on lxplus
• Removing root_numpy from the code and replacing it with uproot