Update the FarBackward beamline, magnets and taggers

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
- [x] Separate the code defining the backwards taggers from the vacuum. #968
- [x] Move beamline definition into single file, separate from magnets. #968
- [x] Address the overlaps and stuck particles in the epic_ip6_extended configuration #903
- [ ] Revisit minimal changes required for update after cryostats added #890
- [x] Add new magnet geometry code based on drawings (https://indico.bnl.gov/event/29346/) #978
- [x] Adapt beamline tracking code to allow non-circular cross sections (antechamber) #989

Required for efficient changes to the geometry to be made and effects understood.
- [x] Retraining of neural network on benchmark failure https://github.com/eic/detector_benchmarks/pull/171
- [x] Update of onnx model attached to new PR for updated geometry https://github.com/eic/detector_benchmarks/pull/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