extended testcase for SubFlow

new tests:
-> verify the id of the original and save/loaded flow is identical
-> verify the id of the original and save/loaded subflow is identical
-> run the flow after save/load and verify the result is OK

code changes to pass these tests:
-> load_flow_from_database() calls add_element() so the special case
   handling of InputData and OutputData is also done in load from DB

remove subroutines from the TODO list, as I believe it's done