Work to get BDT training dataset and Z Peak Data produced

[gordonwatts]

This is a massive merge request. Sorry!! Touches 47 files. Some of the notable things it does:

• There is now a tests directory where you can put all tests
• Lots of tests were added to track the xAOD background work
• Some tests were added for parsing the foreground stuff
• The way represntations in the xAOD backend was upgraded - there now values and sequences, and sequences can be of other values or other sequencse (you can have an array of arrays).
• Simple math functions are now supported (abs, sin, etc.) in the back end (translating directly to C++).
• More binarly operators are supported
• The .First operator is better supported (fixing bugs connected to this prompted the revamping of the backend).
• There is a rudamentary type system now, and if you try to call eta().pt() it will complain that double won't do that. When you call something that returns a complex structure (like prodVtx on TruthParticle), you'll need to add a line so the system knows how to reason about types. Mostly it trusts you just want to make doubles.
• Added lots more event collections.

[masonproffitt]

Something changed in Select that broke my stuff, but apparently the fix was as simple as one line (https://github.com/masonproffitt/BDTTrainingAnalysisLanguage/commit/e904700bc712e15343fa6530ae800a4f2fc9ca6b). I can merge that when this goes in. Everything else looks fine for me.