Implement L1 and HLT prescale sets and corresponding file and db loaders
C++ representation of L1 and HLT trigger prescales sets, and loader classes for reading them from json files and from the trigger db.
While the infrastructure (loaders and prescale set definition) is final, note that the final content of the HLT prescale information itself will still have to be concluded on. For now we need to have the infrastructure in place to test the prescaling mechanism in the technical runs.