* Move repository, cache, and fetcher to logic package
* Move citations model to model/citations/semanticscholar package

* Introduce service layer
* Rename LRU cache implementation
* Add tests helpers for repository

* Move logic from repository to service
* Refactor repositories
* Update tab configuration

…lation-tab-logic

# Conflicts:
#	src/main/java/org/jabref/gui/entryeditor/citationrelationtab/CitationRelationsTab.java

…scussion)

* Implement MVStore for relations as DAO
* Implement LRUCache for relations as DAO

* Solve task 1
* Implementation of a DAO chain: memory cache and MVStore
* Persist citations as relations to disk after a fetch
* Avoid fetching data if relations are available from MVStore
* Avoid reading data from MVStore if available in memory
* Consume less from network, minimize disk usage