simplify scan meta

• Fix bug: calling scan_meta.add_categories({"mycategory"}) twice would add two categories (because of the enum stuff, removed that)
• Simplify: remove code duplication between _profile_metadata_gathering and to_dict
• metadata_profiling: optional scan and timing parameters
• metadata_profiling: NaN when metadata generator fails (e.g. because required scan object is not provided)
• default category timing is START (stays the same) but default metadata gathering is now None (used to be START). None means it gathers all metadata