Tuning of the spin-orbit coupling strength: setting this value to zero turns off spin-orbit terms in
the Hamiltonian, and setting it to one corresponds to full spin-orbit.
This variable is parsed both for RelativisticCorrection = spin_orbit and for
fully_relativistic_zora. In the latter case it scales the ZORA spin-orbit term only,
so SOStrength = 0.0 reproduces scalar_relativistic_zora exactly. This is
the cleanest way to separate the scalar-relativistic and the spin-orbit ZORA contributions
within a single code path.