Maintenance on the generator module

[jm-glowienke]

• Removed target_classes feature, not needed for EIOPA graph
• Due to no target_classes removed the prepare_generator_query function
• Removed use_resources_dump
• Removed lot of warnings and errors due to PEP formatting requirements
• Added check when reading in of templates whether variable(s) of generator_query and placeholder align
• Removed old and redundant code
• Removed file_mode, data files are always created in append mode. Since the ID is unique everytime, this should not be a problem
• Added more comments and docstrings

Closes #19