Match computation and documentation of tau_d

[beutlich]

refs #2247, Q1: Match computation and documentation of tau_d

[beutlich]

Commented by @MartinOtter

Bode codes (the original one and the proposed change) are correct. The original code is correct because:

• tau_d is a protected (auxiliary) variable.
• tau = if abs(b) <= bEps then ... else if phi_diff > bMax then ... else if phi_diff < bMin ... else 0;
  This means that tau = 0 in the backlash region, even tau_d is not zero in the backlash region.

I do not see a reason to change a correct model.