HltEfficiencyChecker: chain HLT1 and HLT2 so we can work out a total HLT efficiency

FYI @rmatev @sstahl @mvesteri @gligorov

This feature has been requested by at least a couple of people now. @mkenzie put it very nicely to me today (quoting):

It's also hard to tell how much better you can do. My Hlt2 line is currently 50% efficient on C.R.C, but Hlt1 is only about the same. So is my line actually 100% on what it will eventually see?

You can get a HLT1 dst and manually filter out those events that fail your HLT2 lines, but it would be good if we could just chain HLT1-HLT2 jobs together in HltEfficiencyChecker, save decisions from both to the tuple and the construct e.g. HLT2 efficiency with an HLT1 line in the denominator.

In principle this doesn't sound too difficult to do, but not sure how much it would break things in practice.