Fix QueryBuilder.count for storage backends using sqlite

The storage backends that use sqlite instead of PostgreSQL, i.e.,
`core.sqlite_dos`, `core.sqlite_temp` and `core.sqlite_zip`, piggy back
of the ORM models defined by the `core.psql_dos` backend by dynamically
converting to the sqlite equivalent database models.

The current implementation of `SqlaGroup.count` would except when used
with an sqlite backend since certain columns would be ambiguously
defined:

    sqlite3.OperationalError: ambiguous column name: db_dbgroup.id

This is fixed by explicitly wrapping the classes that are joined in
`sqlalchemy.orm.aliased` which will force sqlalchemy to properly alias
each class removing the ambiguity.

Author: sphuber

Diff for: aiida/storage/psql_dos/orm/groups.py

@@ -116,8 +116,11 @@ def count(self):
 
         :return: integer number of entities contained within the group
         """
+        from sqlalchemy.orm import aliased
+        group = aliased(self.MODEL_CLASS)
+        nodes = aliased(self.GROUP_NODE_CLASS)
         session = self.backend.get_session()
-        return session.query(self.MODEL_CLASS).join(self.MODEL_CLASS.dbnodes).filter(DbGroup.id == self.pk).count()
+        return session.query(group).join(nodes, nodes.dbgroup_id == group.id).filter(group.id == self.pk).count()
 
     def clear(self):
         """Remove all the nodes from this group."""