Merge branch 'f-versioning' into 'dev'

# Summary

Introduces versioning for entities.

# Focus

Important classes/changes:

* `src/main/java/caosdb/server/entity/Version.java` - the class that represents an entity version. The XML Serialization is done by `src/main/java/caosdb/server/entity/xml/VersionXMLSerializer.java`. Parsing is not necessary as for now.
* `src/main/java/caosdb/server/resource/AbstractCaosDBServerResource.java` - In this class the requested entities are parsed from the last URI path segment. Until now, we assumed, that everything which is not an integer is a name. Therefore we had two  separate getters for requested ids and requested names. Since this assumption is not true anymore, we only have one. This results in changes in several other classes (grep for that method).
* Maybe not so easy to understand: `retrieveValidSparseEntityById` in `src/main/java/caosdb/server/jobs/Job.java`. This is a convenient method to get the entity from the container (if present) or from the database. See the code comments and please come back to me, if that is not sufficient.