An implementation of the parallelized General Triplet Track Fit for GPUs

The General Triplet Track Fit (GTTF) is a generalization of the Multiple Scattering-only Triplet Fit to additionally take hit uncertainties into account. This makes it suitable for use in collider experiments, where the position uncertainties of hits dominate for high momentum tracks. Since the GTTF is based on triplets of hits that can be processed independently, the fit is particularly suitable for acceleration with parallel hardware such as GPUs, and can therefore be used for fast track fitting in online reconstruction. The fitting algorithm is validated with a standalone implementation, using a minimal simulation. A first implementation of the fit is also done in traccc, a demonstrator tracking chain designed for hardware accelerators under the umbrella of the ACTS track reconstruction framework. The implementation in traccc is studied using the OpenDataDetector.