Explicit content style maps for reproducing datasets and manual control

[JohannesTheo]

This change adds the following features:

• the randomly chosen styles will be saved in a content_style_map.json file.
• this mapping can be used to reproduce datasets or to create explicit mappings manually. (we need this for our research)
• the default behaviour is not changed, only the style map will be saved in addition to the images.
• the rglob search with nested directories will still work as expected since we convert content_paths and style_paths to dictionaries for file_name to path lookup.