Make the minimal smoothing length used by SWIFT to match the Gadget definition...

Make the minimal smoothing length used by SWIFT to match the Gadget definition of the kernel support being a fixed fraction of the softening kernel support and not use 'h' directly.

src/hydro_properties.c

@@ -332,8 +332,13 @@ void hydro_props_init_no_hydro(struct hydro_props *p) {
 void hydro_props_update(struct hydro_props *p, const struct gravity_props *gp,
                         const struct cosmology *cosmo) {
 
-  /* Update the minimal allowed smoothing length */
-  p->h_min = p->h_min_ratio * gp->epsilon_cur;
+  /* Update the minimal allowed smoothing length
+   *
+   * We follow Gadget here and demand that the kernel support (h * gamma)
+   * is a fixed fraction of the radius at which the softened forces
+   * recover a Newtonian behaviour (i.e. 2.8 * Plummer equivalent softening
+   * in the case of a cubic spline kernel). */
+  p->h_min = p->h_min_ratio * gp->epsilon_cur / kernel_gamma;
 }
 
 /**