Fix planetary tiny dt

Avoid way too small timesteps from when a far-away particle with the max smoothing length still gets a non-zero changing-kernel term causing huge accelerations.

src/hydro/Planetary/hydro.h

@@ -601,8 +601,8 @@ __attribute__((always_inline)) INLINE static void hydro_prepare_force(
   /* Compute the "grad h" term */
   const float rho_inv = 1.f / p->rho;
   float rho_dh = p->density.rho_dh;
-  /* Ignore changing-kernel effects when h is h_max */
-  if (p->h == hydro_props->h_max) {
+  /* Ignore changing-kernel effects when h ~= h_max */
+  if (p->h > 0.9999f * hydro_props->h_max) {
     rho_dh = 0.f;
   }
   const float grad_h_term =