Create new Kernel-based Simulation Method

[rkierulf]

Feature Request

KomaMRI currently uses an array-based approach when simulations are run on the GPU. It's likely that a kernel-based simulation method using KernelAbstractions.jl would run faster, since some parts of the run_spin_excitation!() and run_spin_precession!() methods can be further parallelized (for example, xt, yt, and zt would not need to be computed sequentially here.