Before this commit, the MySQL connection stayed open during the whole lifetime of the Admidio object. Because of that, new data in the database could not be acquired by the refresh() method. Furthermore, the object would crash when the connection got lost.

Now, the connection is created at the start of refresh() and disconnected after refresh().

This argument was redundant, since it is also a member of the Admidio class, which is also an argument of Group.