RF implementation with GPU acceleration

[cncastillo]

For now, the file Simulator.jl has a dummy function run_spin_excitation_parallel that has to be developed. It is inserted in the function run_sim2D_times_iter (to be renamed):

@showprogress for p ∈ parts
    if is_RF_on(seq, t[p])
        M0 = run_spin_excitation_parallel(obj, seq, t[p]; M0)
    else
        S[p], M0 = run_spin_precession_parallel(obj, seq, t[p]; M0)
    end
end

[cncastillo]

The function run_spin_excitation_parallel is already implemented, but can be faster.