Correction for torsion angles asymptotics

There is a discontinuity problem for small bond orders in the expression for torsion angles and conjugation contributions f(BO). These terms correspond to expressions for f₁₀ (eq. 10b) and f₁₂ (eq. 11b) in the original ReaxFF paper. The new expression for each term in f₁₀ is \(\left(1 - e^{-2 \lambda_{23} BO^2} \right)\) and in f₁₂ the new expression is \(sin(\frac{\pi}{3} BO)^4\). The new expressions ensure correct asymptotic behavior for the dE/dBO for BO \(\rightarrow\) 0. Using this makes geometry optimizations more stable and improves conservation of energy during MD.

The correction can be enabled by setting the tors13 flag in the control file to 1.