Store attachment as single entry

[frankenjoe]

Closes #275

As discussed in #275 it is sufficient to store the path of the attachment in the dependency file. This applies the necessary changes.

This basically changes the entry for an attached folder path/to/folder with ID attachment and the files path/to/folder/f1.txt and path/to/folder/sub/f2.txt from:

                          archive    ...
path/to/folder/f1.txt     attachment ...
path/to/folder/sub/f2.txt attachment ...

to:

               archive    ...
path/to/folder attachment ...

In addition, audb.Dependencies.attachment_files is removed and instead audb.Dependencies.attachment_ids and audb.Dependencies.attachments are added.

I will leave some comments in the code to make it easier to follow the changes.

[codecov]

Codecov Report

Merging #277 (96e52be) into dev (cf5e730) will not change coverage.
The diff coverage is 100.0%.

❗ Current head 96e52be differs from pull request most recent head 6232a8f. Consider uploading reports for the commit 6232a8f to get more accurate results

Impacted Files	Coverage Δ
audb/core/dependencies.py	100.0% <100.0%> (ø)
audb/core/load.py	100.0% <100.0%> (ø)
audb/core/load_to.py	100.0% <100.0%> (ø)
audb/core/publish.py	100.0% <100.0%> (ø)
audb/core/utils.py	100.0% <100.0%> (ø)

[hagenw]

You also remove audb.Dependencies.attachment_files and add instead audb.Dependencies.attachment_ids and audb.Dependencies.attachments. Can you also add this to the description of the pull request.