Feature/spring clean

[buehlefs]

Remove old unused endpoints.

Add new convenience endpoints:

• Endpoint to get all versions of a specific experiment data resource
• Endpoint to get related experiment data resources (currently, related data must have been created in a step where one version of the data was created or used as an input; data relations over multiple steps are not accounted for)

Add a new header X-Attribute-Metadata to the data download resource, when the data is either of type entity/* (except entity/attribute-metadata) or graph/* and related data of type entity/attribute-metadata is found.
The header contains the download link of the latest attribute metadata using the pre relation type.
This can be used by plugins to automatically parse the entities according to the attribute metadata.