Provide script to convert repository structures

[hagenw]

In audbackend we will introduce a new structure in which files are stored on the backend (as added in #62), changing from

/<folder>/<name-wo-ext>/<version>/<name-wo-ext>-<version>.<ext>

to

/<sub-path>/<version>/<name>

As discussed in #76 it would be useful to provide a script that can convert an existing repository structure to a new one, for example by copying the files from one repository to another one.

When dealing with Artifactory as a backend it might be more safe to first download all files, convert them, and upload them afterwards.

[hagenw]

One problem with the conversion script is that there might be no way to preserve owner and date of the single artifacts. This seems not a big hassle, but those two contain important information to provide some statistics over all databases.
We had the problem before when switching publishing from Gradle to audb where we also had to convert all databases and all had the same owner and publication date afterwards which is still causing some trouble.

[frankenjoe]

With #130 there is no longer a need to convert the repository file structure. We now offer a solution to keep working with repositories that use the old structure.

[hagenw]

I agree, there is no need for a script and we should focus on #131 instead.