Migrate to non-relational DB

The limitations of relational data access and orm leads to a change of plans regarding the persistence layer: MongoDB is used to save the original (from gtf) metamodel as JSON; the core-orm project serves as a handler for project inheritance (if needed)

The limitation: parent-child-relationship in table nonterminal led to the problem, that for loading the whole tree, a lot of queries were required if we rely on Hibernate as JPA implementation; when working with SQL/JDBC immediately this problem could be solved as well; however, we need our schemata as JSON anyway (for transmission between components) ->migrateion

(from redmine: issue id 290, created on 2018-01-22tgradl, closed on 2018-01-22)

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