Pipelet and Query:
- Input:
  - move source_subscriber_index and source_transactions from Pipelet to Input
  - update Query_Tree..route() and Query_Tree_Output..emit() consequently
  - update tests
- Query_Tree_Output:
  - embed Query_Tree in "tree" instance attribute instead of "multilple inherited"
  - export into XS.Query_Tree_Output
  - rename _query_update() into query_update()
- Encapsulate:
  - Update documentation
  - remove __update_upstream_query from "Forbidden methods" as it is now in _output plug