Add ATLAS_i2182381 (SUSY multi-b search)

This is the ATLAS multi-b search that has become the testbed for our ONNX infrastructure.

Includes @agbuckley 's optional-onnx branch stuff to build (or not) ONNX dependant analyses based on changes to configure.

The analysis itself is largely validated (just need to double check a couple of Control Region definitions, I'll get in contact with the original team once we have a runnable branch here; and the 3 Gtb cut'n'count region cutflows are also just wildly off for whatever reason).

The biggest hanging question is how to deal with the onnx file, as it's 4.3MB, and gitlab won't let me push anything larger than 4MB. Exactly where this goes and how will influence how we do data path look up.

There's also still quite a bit of room for streamlining, cleaning up, extra tests etc - still a draft for a reason!

NN regions validation plots