Give a sensible DM softening also for runs without DM

examples/HydroTests/EvrardCollapse_3D/evrard.yml

@@ -31,10 +31,9 @@ SPH:
 
 # Parameters for the self-gravity scheme
 Gravity:
-  eta:                    0.025    # Constant dimensionless multiplier for time integration.
-  theta:                  0.9
-  comoving_softening:     0.001    # Comoving softening length (in internal units).
-  max_physical_softening: 0.001    # Physical softening length (in internal units).
+  eta:                           0.025    # Constant dimensionless multiplier for time integration.
+  theta:                         0.7
+  max_physical_baryon_softening: 0.001    # Physical softening length (in internal units).
 
 # Parameters related to the initial conditions
 InitialConditions:

examples/Planetary/EarthImpact/earth_impact.yml

@@ -44,10 +44,9 @@ SPH:
 
 # Parameters for the self-gravity scheme
 Gravity:
-    eta:                    0.025       # Constant dimensionless multiplier for time integration.
-    theta:                  0.7         # Opening angle (Multipole acceptance criterion)
-    comoving_baryon_softening:     0.003       # Comoving softening length (in internal units).
-    max_physical_baryon_softening: 0.003       # Physical softening length (in internal units).
+    eta:                            0.025   # Constant dimensionless multiplier for time integration.
+    theta:                          0.7     # Opening angle (Multipole acceptance criterion)
+    max_physical_baryon_softening:  0.003   # Physical softening length (in internal units).
 
 # Parameters for the task scheduling
 Scheduler:

src/gravity_properties.c

@@ -151,6 +151,12 @@ void gravity_props_init(struct gravity_props *p, struct swift_params *params,
           params, "Gravity:max_physical_baryon_softening");
     }
 
+    /* Some gravity models use the DM softening as the one and only softening
+       length that exists. So, if we don't have DM (e.g. hydro test or planetary
+       physics), we must have a non-zero epsilon. */
+    if (!has_DM && has_baryons)
+      p->epsilon_DM_max_physical = p->epsilon_baryon_max_physical;
+
     p->epsilon_DM_comoving = p->epsilon_DM_max_physical;
     p->epsilon_baryon_comoving = p->epsilon_baryon_max_physical;
   }