Adding command line option prepare + fix metadata

Summary

This MR introduces the following changes

• Adds a command line option -p or --prepare for the train.py script. When given, only the metadata folder is created and the training is not started.
• Fixing an issue that the model_file, which is the start point, is not properly copied to metadata folder.

Conformity

• Changelog entry
• Documentation
• Development guidelines
• Style guides