Metadata-registry-interface-specifications

Breaking structure changes
* task.json is now an array with tasks, so for one DAG one single file with all tasks within that DAG can be provided. The task json must include the uuid of the DAG.
* physical_entity.json
* added location to indicate e.g. in which data zone the entity resides
* renamed LoGSId to source_identifier
* renamed OARId to application_identifier
* the dag.json has been simplified. You can now only reference task uuids. The task uuids have to be listed in a task.json.

Non-breaking structure changes
* physical_attribute
* new attribute: datatype

Structure changes
* Attribute json is now list of attributes for one entity
* Introduced "meta" and "meta_version" in all schemas
* Added "version" and "uid" to DAG, job and Task (mandatory)
* Added "description" to DAG, job and task (optional)
* Job structure changed

Examples and Unit tests
* provided examples for DAG, job and task
* Added unittests for DAC, job and task

* Introduction unit tests