Adding metadata information to training file

Summary

This MR introduces the following changes

• Adapting function name from GetVariableDict to get_variable_dict.
• Removing some useless self.
• Adds metadata information to the final training file. For the jets and the tracks, the variables are now shown as a list with the correct order (corresponding to the columns of the variables in the file) when running h5ls -v on the file. This now looks for the jet and track variables like this:

Relates to the following issues

• Closes #70 (closed)

Conformity

• Changelog entry
• Documentation
• Development guidelines
• Style guides