allow negative time_step / reverse orbit propagation

src/orbits.cpp

@@ -238,7 +238,7 @@ getPropagatedTrueAnomaly(system_2body sys,
 	double initial_anom)
 {
 	// NOTE: 'pause' simulation when time_step is set close to 0
-	if (sys.time_step < 1e-8)
+	if (sys.time_step < 1e-8 && sys.time_step > -1e-8)
 		return initial_anom;
 
 	double ecc_anom = getPropagatedEccAnomaly(sys, initial_anom);