Refactor the HltEfficiencyChecker analysis scripts

I wrote the first HltEfficiencyChecker analysis scripts ~5 years ago and there was much room for improvement, particularly in modularization, readability and maintainability. Finally got round to it.

Also:

• Closes #157 (closed) here with a simple extra error message to catch the particular failure.
• Closes #159 (closed) here by preferring double-colon- over whitespace-separated lists.

I took a little time here to try and make extension to sprucing easier when I get around to that (related to #160).

Tests are all passing for me locally.

TODO:

• Get a review
• Check for other docs updates
• Can we easily reduce duplication in places between the two scripts?
• Final post-review check on tests locally