Update TriggerDB python access methods for Run 2, add new express stream prescale methods (ATR-16555)

Fix getChains method, which had been broken by the transition to the Run 2 database. Add new methods
to make it possible to obtain a list of express stream prescales from a given HLT PSK
Also fix frontier access classes for more robust variable type handling.


Former-commit-id: 9e4eda2dd9d0cf915338c483c17054a1aa4a9952