Fixed the Kutta degenerate case

hspm/src/solver.cpp

@@ -84,7 +84,7 @@ double HSPM::solveOffBodyKutta() {
     double _tau_terms = 2*(_a*_b + _c*_d - _e*_f - _g*_h);
     double _const_terms = pow(_b,2) + pow(_d,2) - pow(_f,2) - pow(_h,2);
 
-    if (_tau2_terms < 1e-10) 
+    if (abs(_tau2_terms) < 1e-10) 
         return -_const_terms / _tau_terms; // If the quadratic equation is degenerate, we can solve it directly
 
     // Solve the quadratic equation