Draft: Add magnetic fields and curved tracks

Track models that include curvature are needed for the correct reconstruction of testbeam data with a magnetic field. I added a completely new track model HelixTrack which fits a helix to the measurement points, similar to the StraightLineTrack without magnetic field. The HelixTrack is able to perform a straightline fit if no magnetic field is given, such that this new model could also fully replace the StraightLineTrack model. Additionally I implemented the curvature from the magnetic field to the GblTrack model. TO DO:

• Implement user definable magnetic field parameter
• Add HelixTrack model
• Add curvature to GblTrack model
• Add correct measurement error handling to helix fit
• Add documentation

If the HelixTrack model should replace the StraightLineTrack model, also the other models and modules need to be checked and adapted (e.g. Multiplet).

I tested the current implmentation with some very basic tests using simulated data with 250 GeV protons and a 3T magnetic field along the x-axis. So far HelixTrack and GblTrack give very compatible results. More thorough test still need to be done (magnetic field orientations, alignment, etc.). Help is very welcome for this :)

Cheers, David