3) Benchmarking of the Tunes versus particle amplitude, with
sextupole ON.
The third step is to benchmark the dependence of a test particle tunes
from its amplitude when the sextupole is on.
The calculation of the tune is performed as described in the step 2).
In order to visualize the island not to far from the bunch center
we take the tunes: Qx = 4.338, Qy = 3.2.
Parameter used.
Note that the island is not visible because to far with respect to the
range explored: in order to see the island we need to take a tune farer
from the resonance, this is done in the next step.
The particle amplitudes are meant as defined in Step 2.