Moved logic to create an ad-hoc entity graph into Jpa21Utils to avoid having to create a PersistenceProvider instance just for the sake of creating such a graph. Moved test code for that into newly introduced Jpa21UtilsUnitTests.
JpaEntityGraph now uses an unmodifiable List instead of an array to prevent having to deal with modifications. Jpa21Utils now uses Collection sorting and reversing to be able to use the for-each loop.
Polished imports, JavaDoc and @since tags where necessary.
Original pull request: #140.
70ad616c4c