fix nx/ny again (values were reversed)

src/simulation/Simulation.cpp

@@ -4534,8 +4534,8 @@ void Simulation::update_particles_i(int start, int inc)
 							// clear_xf, clear_yf is the last known position that the particle should almost certainly be able to move to
 							nxf = clear_xf;
 							nyf = clear_yf;
-							nx = clear_y;
-							ny = clear_x;
+							nx = clear_x;
+							ny = clear_y;
 							// Look for spaces to move horizontally (perpendicular to gravity direction), keep going until a space is found or the number of positions examined = rt
 							for (j=0;j<rt;j++)
 							{