[1D] Improve finite-difference Jacobian

Preserve the sign of elements of the solution vector when perturbing them in
order to avoid triggering discontinuous behavior as the cross zero (which is
caused by the way rate evaluation is handled for negative species
concentrations).

src/oneD/MultiJac.cpp

@@ -56,9 +56,13 @@ void MultiJac::eval(doublereal* x0, doublereal* resid0, doublereal rdt)
     for (j = 0; j < m_points; j++) {
         nv = m_resid->nVars(j);
         for (n = 0; n < nv; n++) {
-            // perturb x(n)
+            // perturb x(n); preserve sign(x(n))
             xsave = x0[ipt];
-            dx = m_atol + fabs(xsave)*m_rtol;
+            if (xsave >= 0) {
+                dx = xsave*m_rtol + m_atol;
+            } else {
+                dx = xsave*m_rtol - m_atol;
+            }
             x0[ipt] = xsave + dx;
             dx = x0[ipt] - xsave;
             rdx = 1.0/dx;