Add ability to chain HLT1 and HLT2 in 1 HltEfficiencyChecker job

FYI @rmatev @sstahl @mvesteri

This MR adds new functionality to HltEfficiencyChecker so that you'll be able to ask questions like

• "What is the efficiency/rate of my HLT2 line given Hlt1XLine (and Hlt1YLine) and Z denominator?"
• "What is the efficiency/rate of my HLT2 line given events passed the Z denominator and passed any HLT1 line?"

which has been requested by a few people recently.

TODO:

• Build the user interface
• Add a new test for this
• Remove --level analysis arg from all examples after #12 (closed) was addressed.
• Update the docs to explain the new features (Moore!1092 (merged))

Closes #24 (closed)

Also closes #12 (closed), as this became a problem for running the analysis scripts with both HLT1 and HLT2 branches in the tree, so I fixed it here.

Requires Moore!1092 (merged) for updated documentation.

Many thanks go to @mkenzie and @mmonk for helping to respectively build and test this.