Update the FarBackward beamline, magnets and taggers

[simonge]

The Low-Q2 Taggers and beamline geometry needs a significant overhaul. This issue intends to keep track of the various steps required and will probably see additional steps added along the way.

Shouldn't effect current detector setup - just prepares for changes

• Separate the code defining the backwards taggers from the vacuum.
• Move beamline definition into single file, separate from magnets.
• Address the overlaps and stuck particles in the epic_ip6_extended configuration Fixes stuck particles in far backward hadron beampipe #903
• Revisit minimal changes required for update after cryostats added Add parameters needed for 6.3 lattice and magnet visualization. #890
• Add new magnet geometry code based on drawings (https://indico.bnl.gov/event/29346/)
• Adapt beamline tracking code to allow non-circular cross sections (antechamber)

Required for efficient changes to the geometry to be made and effects understood.

• Retraining of neural network on benchmark failure Beamline neural network reconstruction training detector_benchmarks#171
• Update of onnx model attached to new PR for updated geometry Close the loop on retraining Low-Q2 onnx models detector_benchmarks#201

Updates required to geometry

• Exchange current drift vacuum with beampipe+antechamber based off CAD (https://indico.bnl.gov/event/29346/)
• Update beamline layout and magnetic fields. (https://indico.bnl.gov/event/29346/)

Further planned updates

• Split tagger readout to match currently proposed design
• Make layer materials more realistic
• Include 2D field maps for dipole magnets