Constrain list of exported functions

[philipp-baumann]

Currently, we export a lot of functions that are more for internal use. While I think it's good to document them with roxygen2 comments, it is a bit too much to export in the namespace because users would never use most of the helpers. This makes the documentation much more focused.

I would suggest to only export what a user might call for core functionality or exploration:

• read_opus_raw()
• parse_opus()
• read_opus_impl()
• read_opus()
• new wrapper and helper functions for extra functionality like extract_metadata() [tbd]

@ThomasKnecht , agree?

[philipp-baumann]

closing since the last feature, extract_metadata() is already in other issues.